It's pretty simple, but the gameplay is pretty good. Controls are responsive, and a one-hit-kill arena (minus the elite 'brute' enemies) is a very fast-paced and fun game. It's more about unlocking the new gear than progressing in the waves (although there is some of that, too) and that forces you to try different and riskier things than would otherwise be warranted. I think that is a good decision, since otherwise the player might get stuck in a rut of using the same equipment at all times, which eventually makes the game boring.
