I've been shooting with the Sigma 8-16mm f/4.5-5.6 for the last 3 months (on my K10D) and I am quite happy with it. The reviews gave it positive reviews, noting there are some minor compromises because it is ultra-wide.
As with all wide angle lenses with dome-like front elements, you will see more lens flare from nearby light sources than with other lenses. I also have occasionally seen a bit more tendency to have purple fringing than with my primes... but I can't quite pin this down to where and when. It's rare, and I think it occurs mainly when I'm slightly off with my focus on long exposure shots at night... i.e. user error in combination with the weak low-light focus capabilities of the K10D.
As for getting a faster lens with larger aperture, I wonder how useful that would be for architecture shots. When I am shooting 110 degrees in tight areas, there is generally a great amount of variation in distance to the subject... and I want a small aperture and the resulting large DOF to keep everything in focus.
FYI, here are two shots from the same place, one at 8mm and the other at 18mm:
http://www.flickr.com/photos/bats1234/4756694820/ This shows you how much greater a view you get with the extra 10mm.
Bob