Another vote for fixed. When my bike was a 8 speed, I was always fiddling with adjustments, but of course the crappy grip shifters were pretty bad. Thumbies would've been better in my case. But I digress. I wore out the casette and bought a new one and then promptly messed up on the adjustment of the derailer and now I have a wheel that is missing spokes and needs a rebuild. So for me fixed gear means alot less maintenence. I can ride it in any weather and as long as my chain tension is set correctly, I have nothing to worry about. My town is pretty flat though, compared to some, so YMMV. For commuting though, I can't think of a more durable setup.

I also built my fixed gear out of an old rock hopper from the late 80's. Steel is real, 26" wheels mean bomb proof durability, even in 32 spokes, and I can run everything from 1.25" slicks to 2.125 knobbies (if I take the fenders off). It is very versatile. See pix at http://rykoala.org/ryko/fixedgear.html, although they're a little outdated. I've since added freddy fenders and wrapped the ugly bar-ends in inner tube.