Overkill surprised me all of a sudden with it's recent release and price. The game is free and I would argue that it is a valuable game. It is a basic shooter with four levels and 30 challenges on each level. There are tons of upgrades for each weapon and alternate fire modes.
The weapons are all well illustrated and the enemy targets are interesting, challenging and constant. One trouble I found was the reload time on each weapon seemed slow. Sometimes this forces you to shoot with exact precision and retry levels constantly.
I highly recommend this game. It is challenging, rewarding and free! Grab it now and have a blast collecting and upgrading all the weapons.