Awesome game!
There were a few disappointments, especially when I bought a new gun and found I managed better with the previous one... In particular the rebound ones almost never hit anything indirectly for me. Could be nice to try out those guns before buying them, or maybe switching between them or between two selected ones.
Otherwise, I liked it that you could invest your earnings into guns or shields, the special skills were also nice but I got to maximal level pretty quickly. Enemies were well varied and challenging.
I myself would have liked a decent storyline better, although its meaninglessness did make it funny :) Overall, excellent!