I owned the Canon for years, and my copy was showing its age. I bought the Sigma to replace the Canon but ended up selling it. The Canon was clearly better wide open, and especially with a 1.4x teleconverter. I also found the focusing more reliable on the Canon, not that the Sigma was bad. To be honest, the Sigma is a great lens and rationally may have been the sensible choice, but there was just something magical about the Canon. That lens was my biggest regret when I moved to micro 4/3. A couple of provisos:
- The above comments refer to the non IS version. A friend had the IS version and it wasn't quite as good wide open.
- Reports are that the Sigma has better bokeh (it wasn't a major consideration for me).