As far as ultrawide angle goes, the Tarmon SP 17mm f3.5 performs quite well, at least on the NEX-6. Adam said that it didn't work quite as well on the NEX-7, so I guess the sensor density has quite an affect on the optics. I tried a different copy of this lens on the 5D classic and the image quality was quite good. Perhaps I should give it a go on my 5D II and see how it compares to the NEX.
Lenses this wide tend to have problems with distortion and edge sharpness; even the Zeiss Distagon 21mm f2.8, regarded as one of the best wide angle lenses, has distortion issues, though it's sharp as heck. I find the Tamron 17mm f3.5 is well controlled on the distortion front, if the camera/lens is not tilted, but it's still very hard to keep lines straight.
In all, a nice lens that still performs well with most digital cameras. You may want to reconsider if you have a high density sensor like the NEX-7, or D800.
All pictures below were taken with the Sony NEX-6 and Tamron SP 17mm f3.5 Adaptall.