The Tamron SP 17mm f3.5 is sort of a controversial lens. Copy variance seems very high. The last one I had wasn't very good on my 5D, but the one I had before that was better (at least on the 20D). This lens is often pitted against the likes of Olympus Zuiko 18mm f3.5, Tokina/Vivitar 17mm f3.5 and the Nikon 18mm f3.5, and to lesser degree, the Canon FD 17mm f4 and Minolta MD 17mm f4.
The Zuiko, without doubt, is the most expensive of the lot; often costing 3X or more of that of the Tamron 17mm f3.5, and it vignettes much worse than the Tamron, and has a bulging front element and kludgy lens hood, but possibly has better distortion control. You can see the test results of the older version with built-in filters by Modern Photography against other similar lenses by OM, Minolta & Nikon here. From the test, we can see that the Tamron should be a good performer.
Be My Valentine - Canon 20D & Tamron Adaptall 17mm f3.5 . Click for larger.
Gooderham Flatiron - Canon 5D & Tamron Adaptall 17mm f3.5 . Click for larger.
Tiff Festival 2012 - NEX-5N & Tamron Adaptall 17mm f3.5 . Click for larger.