It's a really fun game. The weapon diversity is good, and the gameplay is smooth. Good job ! I just kept playing, trying every weapon and upgrading them. It simply work.
The music is very catchy, I liked it. At some point, I could have sworn that I was firing in rythm with it, it felt awesome :D
The downside is that the difficulty isn't scaling as much as I'd have hoped. Depending on the weapon, the game become much easier after some level. I also think that the camera is really close, wich is fin by itself, but it may have stopped you from adding some interesting scaling mechanics, like much faster enemy, or bullet to dodge.
Overall, solid game with a very efficient game feel. I enjoyed playing it !