It's a pretty arcade shooter, one that makes as good a use of the iPhone's touch screen as it can with a "floating gun" control scheme, which uses virtual joystick to aim and has you shaking the phone to reload.
While Overkill tempts you to grab it by virtue of the fact it's a free download, be warned, it's not actually a free game: like an increasing number of mobile games these days, it allows you to get started playing for free, but if you want to advance further in the game, you'll need to buy yourself some better guns (or engage in a slightly convoluted scheme where you download selected apps from an advertiser and leave them running in the background).
If you're cool with micro-transactions, using the "free" status of the game as a demo, you'll probably have a blast with Overkill. If you're the kind of person who likes to spend $1-2 for a game and be done with it, though, be warned!
Overkill [App Store]