I also think they seem somewhat satisfying and increase the replay value. I've not so many as some of you, just about 5k.
The next release of my game will have OF achievements. I think it's important to spread them across the game: some are easy ("finish the tutorial levels", or "hit your own cannon"), some take 30-60 minutes of playing, while others are there for the experts to strive for. I know plenty of gamers with 100k XBox Live points so I know how addictive they can be.
