Leftovers - well not quite! I have leftover filling though, so I'm going to make some more this weekend! I have potatoes and cheese ones to make still....cicero you are welcome to drop by and sample them!
MrsPink - yes, I have had the problem of the pierogi opening up while they're being boiled as well. I read that the main reason for that is that the dough tends to get dry when you're filling them and sealing them...so the best thing to do is try to do that part ASAP after the dough is rolled out. Also make sure to fold over the openings and even pinch them shut, maybe with a fork, to close the seal even more.
And yes, getting the dough quite thin helps too - mine was almost too thick last night, but only 1 or 2 opened. I was worried though!