Okay, after playing through levels 1-2 and that repeating one, as well as trying out all of the weapons and stuff, I figure I should take care of this while it's fresh in my mind.
:Are the controls and interface intuitive?
The controls were smooth, and I think they're great. The menu interface was good for a keyboard only game, though mouse support could be nice.
Quick question: Why was "L" the fire button on the WASD configuration?
:is the difficulty too easy or too hard?
Considering these are the first few levels, I'd have to say that the game starts really easy on the first stage. On the second stage I got hit once or twice. On the repeating stage I had to start trying and got hit somewhere around four times. So far I think that for the beginning the difficulty is just right.
:do the rpg mechanics enhance the game?
The light RPG mechanics are pretty interesting, and I think they add a bit of depth and customization to the game.
Also, I noticed a large increase in DPS when you change weapons.
the needle gun pretty much tore through everything much faster than either of the other choices. Was that intentional?
With a base attack of 3.75 I got these stats.
Standard Bullet rifle : 28.5 DPS Price: 20
Needle Cannon: 37.5 DPS Price: 30
Durable Cannonball launcher: 32.5 DPS Price: 25
Seems like there's a pretty huge difference between the choices in comparison to their extremely similar costs.
The rotating of your weapon seemed kind of useless at first, but then I remembered the screenshot in the OP and I thought that was a clever idea. I'm kind of sick of only being able to shoot straight.
:did you find any bugs?
A couple of times I'd hear an electronic-ish sound when browsing through the Parts shop.
Also the game would say I'd have one of a certain part in the Parts shop, but when I'd check the garage it wouldn't be there. To be specific, I can only be sure this happened with the enhanced engine. (It may have happened with another part but I'm unsure of that much.)
I didn't hear any sound other than that occasional beep, so is that the only effect in the game right now?
:did the game run well?
Yeah, it ran smoothly without any major jumps or screwing up, the save file loaded fine as well.
:is it... fun?
Yeah! Once this gets finished I can see this being quite a bit of fun. As is it's entertaining, but I could see it being a lot better later on.