When I first opened up the package, I was suprised to see how small the lens is! It's much smaller than the 12-24mm Sigma, I knew it was going to be a bit smaller, but I was pleased to find that it's quite a bit smaller! I'll post up a size comparison later.
Build quality: Very nice! So far I have owned two Sigma EX lenses (this, and the 12-24mm) and I am very pleased with the quality thus far! The focus ring is nice and smooth, easy to turn but not loose feeling. The lens itself feels very solid, right down to the metal lens hood (perminantly attached). A nice semi-soft case was included as well, thumbs up to Sigma for that!
The auto-focus is not super fast, but it locks on target with no problems, and does not hunt at all before it locks. Just zips right to focus. It's tad bit loud (a small whine sound), but nothing that would draw to much attention or get anoying.
Image quality: I won't do any formal tests or anything, but so far from the images I have shot the quality of the glass and aperture blades seems to be great! I shot a few closer shots at f2.8 (wide open) and the image was nice and sharp! The background blur was smooth and did not look ugly or disspleasing. (Bokeh is only noticable when shooting close-up objects) I didn't notice any chromatic abberations or other anomalies (aside from the obvious fisheye distortion! ). I am very pleased to see that this lens can be used wide open with out fear of a horribly soft image. Much much better than the 12-24!
This lens is quite fun to shoot with! Depending on how you angle the lens, the fisheye effect can be very noticable, or it can be hardly noticable. This lens is definantly not an *every day* lens because the fisheye effect can probably get boring & over-used pretty quick, but I'm sure that it will see quite a bit of use for those times when a good *fishy* shot is needed or when I just need to crack a smile on my face
I put the lens on my film body (EOS-1) and the fisheye effect is quite a bit more dramatic than on my 1.3 crop 1D. The fisheye effect is still very much prevailant on 1.3 crop though, no doubt about it. Anyone with a x1.6 crop camera will not notice quite as much fisheye effect, but I'll bet it's still there real good on x1.6 as well.