When I first saw Rozen Maiden, I couldn't get the OP out of my head. The more Ali Project I heard though, the less I liked it. They suck pretty hard, but it's not like they're the bottom of the barrel. Most other OP/ED music is entirely skippable--so painfully generic and boring that I don't even watch it all the way through once.
Theme songs in general are lame. They're only there to take up space.