I just bought it myself, because it was there and I was bored.
So far, my experience has consisted of the following things:
1, dying
2, confusion
One problem is that the tutorial is VERY SHORT. It hardly tells you anything, and then says "Dont worry, you'll learn the rest by dying!" which is always a great omen but not at all unexpected in anything that's whatsoever similar to a roguelike.
I havent played it all that much yet, partly because it's late, but will spend some more quality time with it tomorrow, but from what I hear, there's a TON of content in it. Quite alot of game for the price it's at right now. I bought the game and got all the DLC and stuff. I notice some characters from TF2 are in there, because.... because TF2, those characters being EVERYWHERE seems to be half the reason TF2 exists. Gotta love Valve for that one, really... I only messed with 2 of the characters though so far since I barely even know what's going on.
One thing I REALLY like though is the ability to pause and still issue orders. I think every game should do this, really, when it's RTS-ish. I dont have to worry so much about inaccuracy with the mouse (particularly right now, as the thing is damaged) or pain from wildly waving it around. So that's good.
I'll post more impressions later. But this one looks really promising. And impossible. I mean, seriously, I died on floor 1...