Tokina made an excellent decision here, making a fisheye lens with wider applicability. Fisheye lenses are fantastic for busy scenarios and work great for kids. Having the zoom, allows to control the amount of "fishyness" in the image, hence also giving normal looking images without the need to change lenses. This is my favorite lens in parties, mostly on canon 40D (1.6 crop sensor).
Build quality is excellent. It is small enough to stay in the bag 'just in case', the cap stays firmly on. It works both on crop sensor and full frame cameras.
The Canon 15mm indeed gives better image quality, and f/2.8 is better than f/3.5. Both of these are minor issues though. The flexibility of the zoom outweighs that big time.