I've played it for a bit and do enjoy it. Bought the thumb strap for better control and I do like that method. As close to keyboard and mouse as you're going to get on a console-ish system.
I agree with everyone that it just has this open ended feel once you finish a boss. It's also never clear to me which relics I have, which ones have been stolen, and if they've been stolen, where to go to get them.
Oh, and while the idea of an automap is great, it's really hard to figure out because the screen is so small. It's like the map in original Descent, but at least you had more room to play with that map. Anyway, it's much easier to simply do your best to memorize where you're going so you can backtrack after you beat the boss.
I'm a PC FPS junkie, and this game has impressed me. Never thought I'd see an FPS this fun on a handheld.
-HM