WARNING: If you don't like reviews with the odd bit of Yuck Factor (TM) or can't stand reading anything on how to deal with "your monthly visitor", then please don't read this review! You might like one of my other reviews though on such topics as food or booze (no offense intended).
Firstly, let me say that I'm fully aware of the fact that there are already 20+ reviews for this product, but I'm hoping with my review to give a bit of information that isn't found in them and a bit of background on the history of this particular product.
****So what the bejebus is a Mooncup when it's at home?****
A Mooncup is a device that's worn inside the female body at 'that time of the month' to collect the shed uterine lining. Basically, it's what is known as a menstrual cup. It looks a bit like a little translucent sink plunger with tiny holes around the rim.
****Eugh! That sounds horrible!****
That was my first reaction too, mixed with curiosity, but if you sit and think about it for a bit it's not actually that bad, when you take into consideration things like tampons. We wear them inside as well don't we?
****So what's the difference?****
Well probably the biggest difference between a tampon and a Mooncup is that a tampon soaks it all up and a Mooncup holds it until you empty the contents yourself, which, although can be sometimes a bit messy is I think a lot easier than having tampons swinging about! With the Mooncup, you just get rid of the contents down the loo, give it a rinse or wipe out with a bit of clean loo roll and put it back in. Also, whereas you're advised to change a tampon every 4-6 hours (more if your periods are heavier), with the Mooncup you can leave it in situ for between 4-8 hours and very much overnight. And that extra couple of hours can make all the difference sometimes!
Tampons are made from an absorbant material, usually a mix of cotton and rayon. Mooncup is made from medical grade silicon, so is completely washable and doesn't run the risk of shedding fibers inside you, thus greatly reducing the risk of developing toxic shock syndrome (relatively rare illness that CAN BE FATAL caused by staphylococcus aureus bacteria, which normally live harmlessly on the skin and the nose, armpit, groinal and vaginal areas). Because they're reusable, they're much better for the environment and one Mooncup can last for up to about ten years, which is fantastic, considering the fact that the average woman can use over 10,000 tampons in her lifetime .
****Can you get them in different sizes like you can with tampons?****
Yes you can. They come in two sizes: Size A, recommended for women that are under 30 years and had children naturally (ie, a vaginal birth) or are over 30 years old and have given birth vaginally or by Caesarean, or not given birth at all, and size B, recommended for women that are under 30 years old and have either not had children or have had a caesarean.
****What about allergies?****
The product literature states that a Mooncup is suitable for women that have sensitive skin or allergies. It shouldn't promote a reaction due to the medical grade silicon it is made from, although it does say that if you have, or suspect that you have a gynaecological condition, that you should consult your doctor before deciding to use one.
****OK, so are these 'menstrual cups' a new fad?****
No, not at all! There is a website called Menstrual Cups where the author has discovered references to menstrual cups and similar devices going right back as early as 1867, when they were called Catamenial Sacks (sounds even worse, doesn't it?). Apparently the word 'catamenial' means 'menstrual'. This first device was called the Hocket Catamenial Sack and was 'an internally worn pouch like item that was connected to a belt... To empty it you would remove the pouch and reinsert it again afterwards'. I imagine this to have been a similar set-up to the sanitary belts that used to be used to hold sanitary towels in place before the advent of the ones that you stick (literally!) to the inside of your knickers nowadays!
As time went on, various patents were submitted for similar devices made from such materials as rubber and even metal (ouch! That must have been cold to start with!). In 1892 (with a few inbetween) came the Vernier Catamenial Sack which was an extendable one, presumably to fit the individual.
There's actually a Museum of Menstruation and Women's Health that has photographic examples of actual menstrual cups dating back to 1932, when the rubber Daintette was patented.
In 1970 came the Tassaway, which was a disposable cup for women that didn't lick the Eugh! factor of reusing menstrual product. This was also tested for the possibility of causing TSS in the late 1970s and was found not to be a causal factor.
Popularity of menstrual cups was quite poor partly because discussing menstruation was very much a taboo subject, in that advertisers were not allowed to mention certain words, such as "vagina" and "menstruation" and some even refusing to print images of the product. So, because it was something they wouldn't show or couldn't properly describe, the manufacturers were facing an uphill struggle to get their products on the open market. Add to this the fact that in the 1930s tampons and disposable towels were being marketed as the convenient way to deal with periods because of their disposability and the fact that when the earliest cups came out it was seen as highly inappropriate for women to insert anything into their vaginas and very distateful to put something in, take it out to wash and then put back in again.
Anyway, jumping forward a few years to about 2000 sees the advent of Mooncup UK. There's a Mooncup US available too that came out in about 2006, with similar items going under the names of Instead Softcups (disposable devices that came about in 1992), Divacup (common in the US and produced in 2004) and Lunette (2005). 
****Right, back to the Mooncup. Does it hurt or smell when it's in you?****
The answer to both these questions is usually no. Because it's worn inside there should be no odour and the products of your period aren't held against the inside of your body, like they are with tampons. On initial use it can be uncomfortable (a couple of the already existing review talk of discomfort). They may not be perfect for everyone though, because we're not all the same, but in general once the user has got used to them there shouldn't be any problems. When I first used mine I actually found it very uncomfortable, but I think it was because I didn't position it quite right. I'm glad I was at home that particular day! Since then I'm happy to say everything seems to have gone smoothly. The only problem I've really had is sometimes when I've needed to defecate it's tried to escape (because of the way my internal muscles work I think and because it's held in place lower down than a tampon by your muscles). I get over that by holding the stem (I'll explain about the stem next) and the cup in place with toilet roll if I find I need to.
****Is it easy to get in and take out?****
Relatively, but you usually get better with a bit of practice. It has to be sterilised before the first use (the book tells you how) and you have to fold it in a certain way, which sounds quite faffy but isn't really that easy. You just have to be careful not to let it go or it could go pinging across the bathroom!
When it's properly in place it forms a sort of suction seal and to take it out you have to break that seal by either squeezing the base (and getting your finger and thumb in - short nails are essential here! You don't want to run the risk of scratching your vaginal walls!), or running your finger around it to dislodge. This is where you can get a bit of Yuck factor because there may be a bit of period fluid on the outside from before you put it in, but it depends how squeamish you are with your own body as to whether or not you mind doing that. Beware the possibility of sucky noises on removal!
****Right, so what's this stem business you mentioned?****
Remember I said it looked like a sink plunger? Well, a sink plunger has a handle, right? The stem of the Mooncup is a bit like that sink plunger handle with the difference being that you trim a bit off with scissors before you first use it. This is to ensure that the end of the stem is sitting inside you when in use as no part of it should be poking out. You need to get it right because it can make you sore if you don't.
****So how do I do that then?****
There's a very good little book that comes with each Mooncup that tells you how. It also tells you just about everything you need to know about the use, care, storage and everything related to Mooncup use.
****What if there's something I need to know and it's not in the book?****
There's an excellent website which can be found at www.mooncup.co.uk that will tell you everything else you need to know.
****OK, I'm sold! How do I get one?****
You can order them direct from the Mooncup website for £18.99 (current at the time of review writing). This includes postage to anywhere in the world. Alternatively, if you don't want to wait for the postie, you can buy them at many branches of Boots. I don't know if this is all branches, but certainly at the larger ones. They're about the same price there, although I got mine last summer when they were a couple of quid cheaper and I also got a voucher from the Advantage Points machine which got me another £3 off, so it's worth checking it out.
****What do you get with it?****
The Mooncup comes packaged in a discrete cardboard box made from recycled paper. You also get:
*An unbleached cotton bag that ties with either a pink or green ribbon, depending on the size of cup. The bag carries the logo and name 'Mooncup'
*A booklet (like a mini Bible!) written in about twelve different languages
****Shall I just get one then?****
I would say that before you go running out to get one please have a good read of the Mooncup website first and also read a few more of the reviews on here to get a more rounded opinion. Personally I think they're fantastic and I' won't be without mine now.
Also, for a bit more historical background if you want to read it, you can have a look at the site for the Museum of Menstruation and Women's Health.
 - Mooncup box
 - http://labyrinth.net.au/~obsidian/clothpads/Cups_history.html
1) - mooncup.co.uk
2) - Menstruation@Everything2.com
3) - www.mum.org (Museum of Menstruation and Women's Health)
You can also find advice, tips and other women's stories if you have a Live Journal account by joining the menstrual_cups community.
Hopefully it will work second time around. If it doesn't show up, the only reason why I can think of is that my copy/past is carrying over some script that prevents my copying to other sites...
EDIT: So far, so good. Let me know if it decides to disappear again!