In reply to Dr Avid: YEah Ive got one and love it on my 350d. Just had it in INdia with me and great for architecture and landscapes with people/interesting foregrounds. Obviously ultra wide gives a unique perspective and there is fairly heavy distortion especially at the edges when a subject is close to the lens - e.g. someone walking in the corner of the frame will have their arms/body stretched. Build quality on mine is great. Feels solid and IQ is nice, sharp and bright.
Lens is fairly slow but you aren't really gonna be using an ultra wide hand held in low light. composition etc takes a bit of getting used to but now I wouldnt give it up as it gives some unique takes on scenes.