I've had a medium Meluna for about... nine or ten months?... and I never had a leak once I got through the learning curve. But the last three months, I've woken up to a huge amount of leakage.
I'm tossing up whether to invest in a larger-capacity cup, or whether to use backup pads at night for now. I can't really afford a new cup right now, since the things aren't cheap, but a few packs of pads costs the same so that will end up more expensive in the long run. I think it might be best to use up the pads I still have left from pre-cup days and then in a couple months I'll have a bit more money to get a new cup.
But which cup to get? It was hard enough choosing the Meluna! :)