Oh man, this game. I remember LOVING it as a kid and I actually just played it again recently (which is how I got more than half) and it's just okay. Without the missions to keep your objectives varied it would be a heap of nothing, but with them it can be fairly engaging.
This is exactly how I feel about Jetpack Joyride. It was one of my favourite games as a kid, nowadays it just a fun game to come back to from time to time. The main thing for me is to somehow transfer my data from my 6-year old iPod to my iPad.