i have the sigma 12-24 which is very wide on my full frame - for sharpness and obviously distortion it's not great - but for the money it's certainly not too bad... for that width you'll struggle to get anything decent for under £1,000 and they still won't be much better. I've had magazine covers from mine!!
Yes, quite like what I have seen of it so far, liked the feel of the lens also.MPO didn't think sharpness was too bad. Think I will ponder and try others first, before deciding. Just purchased the 28-105mm zoom, I like this lens a lot I have to say.