My name is Matt and I am a fast sharp lens addict, an extremely expensive addiction. I read about this lens on various reviews and determined it would be perfect for the way I shoot. After looking for awhile, I found this lens used on EBay for less than half its MSRP. I was told that it was being sold for a client that took it on safari in Africa and didn't need it anymore. It has a very slight rub mark on the bottom from bracing on a Landrover during the safari. A lot of money for me at that time, but I would do it over again if I had to.
I own a good number of L lenses for my Canon digital bodies and I can truthfully say that this is the sharpest (with the possible exception of my 200/1.8) lens in my arsenal. SigMonster it is. Yes, it is heavy, but I have handheld it for pictures of soaring turkey vultures that came out great. I have taken shots down to 1/90s at 800mm with this lens and at that I could literally could see the reflection of my house in the Goldfinch's eyes. I use it mostly on a monopod without a gimbal head and the weight is not a problem. In fact, I believe the weight and length of this lens actually help with such shots due to the high moment of inertia it presents to damp quick movements endemic to shorter lenses.
This lens is an absolute MUST for birding and photohunting. It has tons of versatility, a wide range and is compatible with teleconverters. It really enables you to reach out from a distance and grab the shots you need. Next step for me is to put it on an equatorial telescope mount and see just how far I can reach out. Jupiter anyone? Seriously though, if you have this lens and do astrophotography there is no need for a Takahashi scope -- you already have optics that rival one. I have read about problems with build quality with Sigma lenses, but I believe that you have the same problems with Canon L lenses as well. I have a Canon 70-200 f2.8 IS USM that I now think is a dud because of this lens.
To sum up, I would not trade this lens for a Canon lens even if a matching one existed in the Canon line. I could not ask for a sharper lens and more verstaility in a single package. My biggest wuandary now is whether to give it's little brother, the 120-300 a try!