I know some people here have said the game didn't "grab" them like Geometry Wars or other addictive shooters, but it's got me hooked. Here's why:
When you beat a level, if you end the level with an upgraded weapon you start the next level with that weapon and get to keep for twice as long as normal. It really gets me to play just one more level each time it happens.
Also, if you collect enough energy in the bonus levels that energy will add to your bar at the start of the next level. I don't want to waste that energy, so I'll play just one more level again.
It's a pretty fun little game. Some levels are very challenging once you hit the mid-20's. I'm on 35 right now and enjoying it very much.
I love the tactile feel of snagging items with the grappling hook, and the weapon power-ups are very satisfying. Definitely worth $7, IMO.