We just repainted our living room and dining room, so I went out and bought one of those little edger tools. It was great for a little while, until paint started leak onto the edger, which then smeared paint on the ceiling.
Lesson learned - it's a pain to tape off a room, but it's a bigger pain to touch-up the ceiling :(
Those sponge edgers are useless as well - give them to the twins to paint with instead.