Summary: I also own a Sigma 18-250 which is wonderfully sharp, and after reading reviews from Canon users who also own similar L series lenses I thought this lens was worth a shot given it's range. It's sharper than some would have you believe but your technique has to be spot on if you want to get sharp images either hand-held or with a tripod.
Pros: Fast / accurate auto-focus, Strong Construction, Sufficiently Sharp
Summary: So far, I used this lens in bright sunny day only with plenty of light, hand held. It produces excellent sharp pictures with great colors even fully extended at 500 mm if subject is at medium distance, I would say, 50 yards; remote subjects come little bit softer, but still quite acceptable. AF is fast and accurate, and can be overridden without switching the lens or camera to manual mode.
Pros: Consistent Output, Durable, Easily Interchangeable, Fast / accurate auto-focus, Strong Construction
Summary: The new 2010 Bigma is a well built lens with a solid feel. I appreciate the extra reach that it offers. Of course, as with all zooms, it does get a little soft at the furthest reaches, but it's still acceptable. Pull back to 400mm and you're getting a sharp shot that you won't typically get from a 400mm zoom. I was concerned about how this lens would perform in shade, and have been very happy.
Pros: Consistent Output, Easily Interchangeable, Nice Bokeh, Strong Construction
Summary: You're going to need at least a Monopod because this Is a big bit of kit but the results you will achieve will by far outway the hassle of lugging It around. Focus Is virtually silent and fast enough to follow larger birds in flight. See photo.