Fantastic.
It's simple, it's fun, it's easy to play, and most of all, it's addicting.
I could go on and on about how much I enjoy playing this game, but I'll just get straight to the point. You took a simple idea and made it fun and enjoyable. It gets hard, but who cares? Once you lose, you can easily restart and try again. No money to worry about, no upgrades to gain or lose, no damned achievements, it's just a simple and fun game.
Now, the lack of depth to this game could go either way. I love the fact it's short and simple, but at the same time I wish there was some sort of leveling system. If a sequel were to be made, so many more things could be implicated. Gun upgrades, speed upgrades, armor, the sky is the limit here. But as far as this simple concept of a shooting game goes, it's flawless. It has enough to make it a full game, and it doesn't lack any necessities. The physics are what really make this game for me. Shooting enemies in their feet makes them fall down, shooting them in the waist causes them to grab the wound and bend over, etc. This game is realistic and fun. The lack of background actually makes it better in my opinion. Instead of it being a saloon or in front of a building on a dirt road, it lets you use your imagination, and it just gives it to you straight: This isn't a game with bells and whistles and pointless features. It's a quick-draw game at it's absolute finest and it shows flashy effects and good graphics aren't needed for a fun gaming experience.
I can't wait to see what's in store for the future. :]