Pretty good game for an intern project. Proving that simple + fun beats flashy graphics + crap gameplay.
Simple to pick up: shoot things, avoid getting shot. Fun powerups. No arcane controls, no learning curve. Doesn't require hours of time investment. Sit, play, and go at your leisure.
Co-Op: Well conceived. A lot of co-op modes involve people who are basically doing their own thing with no teamwork or coordination. In this game, the formation system is very well done and gives a real team dynamic. Weaker players can play "gunner" and let the stronger player do the driving. Powerups can be split between players with a bit of savvy driving.
Single player: Haven't played on insane difficulty yet, but normal was relatively easy. I was always overloaded on health and powerups. Some bosses present, but not enough, and none of them are especially tough.
Final comment: The only real thing that stands out as a negative is that it is short. I would have loved to have seen more bosses and levels. MS finally gets a good marketplace story in the face of numerous flames about microtransactions. I'd personally love to see a sequal, or expansion pack. (I'd do 500 MSP for a fuller game) I could even see it going the way of Geometry Wars and Marble Blast where it becomes a "standard" XBLA title.