I'm not too worried about becoming allergic to the keeper, as most people with latex/rubber allergies actually develop them as children via the use of latex in the health fields though much of that is being fazed out. Latex and rubber are renewable resources too for those who are environmentally conscious enough to buy and use a menstrual cup in the first place.
I personally would like to leave as small a footprint as possible when I'm gone, so even that 'little silicone cup' as another poster wrote would bother me.
Good luck with whatever you choose. I'm having to use tampons while I wait for my new keeper to be delivered and they suck (even the natural fiber non-bleached ones I chose) not comfortable and don't last all day at work.