Remember R-Type, a shooter that basically have you shoot everything coming from the right side of the screen? Overkill is like that.
You can get weapon upgrades between levels, and with 3 different weapons (guns, missiles, special) there is a bit of RPG involved.
By using tilt controls motion is very smooth. Worth trying.
NOTE: NOT related to the vertical 2D shooter from Epic Megagames.
http://www.appbrain.com/app/overkill-%5Bspace-shooter%5D/com.creativem.overkill