After much deliberation I decided to sell my Tamron 200-500mm and have just bought the Sigma 150-500mm f/5-6.3 APO DG OS lens for wildlife photography. Probably like many Canon users I was torn between buying the Canon 400mm f/5.6 or the Canon 100-400mm. If money was no object I would have chosen the Canon EF 500mm f/4.0 L IS USM. Back in the real world, I opted for the Sigma 150-500mm because of the extra reach and the OS system, at an affordable price! This gallery contains some of the first shots I've taken with the lens. First impressions are very positive, the lens seems to be EX standard in terms of build quality (even though it doesn't have the EX designation). The lens is definitely light enough to be hand holdable, and the OS system seems quick, effective and quiet (despite some reports that it is noisy). One thing that is immediately apparent compared to my Tamron 200-500mm, is the bokeh is much more pleasing.

I'll try and post a few more photos over the next few days to give an idea of how this lens handles in real-life in the hands of an amateur photographer.
All of the images in this gallery were taken hand-held, unless indicated otherwise.

08 Aug 2008 Update - After using this lens for 3 weeks

Overall I'm very happy with this lens, its much better than my previous Tamron 200-500mm. In many ways it has surpassed my expectations, for £599 I was expecting it to be soft at 500mm. However, I'm very happy with this lens in day to day use and resolution seems fine to me at 500mm. I've certainly got shots in fading light hand-held which wouldn't have been possible with my Tamron. The only slight limitation I've noticed and this may be a reflection on my technique and not the lens, is that it can take a while to lock on to fast moving subjects (e.g. birds in flight).
However, as most of the subjects I photograph are static, I'm not too concerned about this. Based on my use over the last 3 weeks, I would definitely recommend this lens.