Recently, I sold my old and trusty Tokina 19-35 AF193 lens, because I could buy a Canon 17-40/4 for a very good price.

First of all, the Tokina AF193 is a completely different lens than the cheap Vivitar/Tamron/Quantaray 19-35 lenses you often encounter. A night and day difference actually, both in performance and build quality.

Anyway, looking back and comparing pictures, the Tokina wasn't really worse than the Canon. Granted, the Canon lens adds weather sealing and a tiny bit more contrast, but the Tokina was as sharp as the Canon, and (in good Tokina tradition) built like a tank.

I don't regret buying the 17-40, but I do regret (a bit) selling the Tokina. If you encounter an AF235 or AF193 (which replaced the AF235), it might we worth to give it a second look. Tokina coatings and build quality are great, and optically, this lens is very good.

There is one important aspect where the 17-40 excels; barrel distortion at the wide end. The Tokina is not doing great there, so that's not a good lens for architecture shots. But since those Tokinas often can be found for low prices (including the lens hood), it would make a great vacation or knockaround lens.

Alas, I sold mine.