It depends somewhat on the type of snails you have whether you are likely to become overrun or not. Ramshorn snails (the round ones) and the long, cornucopia-looking ones (I forget the name) will turn from one snail into a zillion snails in no time. They reporduce at a frightening rate.

Apple snails, which get big and are cool, lay their eggs right above the water line. So if you keep the tank full to the top, they are less likely to lay eggs.

I have mystery snails in my tank. I think they are the prettiest. Mine have lavender/purple shells and black bodies. They get pretty big. I have heard they don't reproduce as easily as other snails. And that's a good thing!