I was hunting for a UWA which does not produce distortion, CA problems, must be sharp, good built quality and most importantly ... affordable. So while waiting for Canon to release their EF 8-15mm f4L, I got my hands on the Sigma 8-16mm f4.5-5.6 HSM instead. This is a crop body lens with a EF mount, yes you can mount it on your 5D, 5DMkII but you will have to shoot at around 13mm to avoid the vignet/black borders. But on a crop body like the EOS 7D, this lens is WIDE~!
I was expecting crazy distortions like the previous Sigma 10-20mm that I've used before or even the Tokina 11-16mm but I was really surprised that all of the vertical straight lines, remained as vertical straight lines. Next test was to shoot into a heavily backlit environment, zoom in 100% and wow! no CA issues! How can this be for such a wide angle lens? What magic has Sigma put into this? Well basically, they've put in 4 pieces of FLD glass which is supposed to deliver similar quality images as those produced by Fluorite glass. For the next 2 days of obtaining this piece of glass, I put it through real life shooting situations, from landscape, architectural to a pre-wedding photoshoot (and yes I didn't get any distorted bride/groom).
Color reproduction, sharpness, focus speed, built quality are really off the scale on this lens. If you're a techie and want to know more about the technical specs of this lens, just go google and read some of the articles online.
I've attached some of the photos taken with this lens and you can find more of them in my flickr account : here
Take a look at the pillars/doors of the temple photo and you'll get an idea of what I meant by vertical lines being straight.
Lastly, would I recommend this lens? With the new FLD glass in this baby, most definitely :)