I played with a Sigma 30mm f/1.4 lens at the camera shop this weekend as well, and ultimately the slightly faster autofocusing and smaller body (literally, half the size of the Sigma) won me over more than the technical merits of one lens vs. the other. I'm shooting on a D50 body, so the lack of a motor in the lens was an asset, not a drawback.

I've only had a few chances to shoot with it, and so far I'm finding that it reminds me of the Canon 50mm f/1.8 I used to shoot when I had a 35mm body (and which I still swear by if you're shooting film on a Canon body). I've found it to be sharp, clear, and overall very, very well suited for the sorts of photos I like to take. I may still pick up that Sigma lens (or the upcoming Nikkor 35mm f/1.8 AF-S DX lens, which while only an incremental improvement, is priced pretty competitively) down the line, but I don't think 3 fractions of an f-stop on the Sigma or the middling focal length disparity between the Sigma and the Nikon are make-or-break items for me right now.

If you find yourself with a chance to pick the Nikkor 35mm f/2.0 AF-S lens up cheap (<~$250US or <~$200 grey market) I'd recommend it as a nice, compact walking-around lens.