As someone who bought one noted, it does not dovetail with other f/2.8 focal lengths, so one should ask how it fits into their overall scheme of things before buying it and then asking what lenses do I buy to go with it. That focal lenght definitely does not mesh with Canon f/2.8 lenses, I'm not sure it does with Sigma either, and Sigma does not really have top quality lenses that form a 3 or 4 f/2.8 lens system.
Not exactly dovetailing, but that unique range is the genius of the lens, according to some lens critics (TDP, Lens Rentals, et al). I am not telling you anything you don't already know in saying most people don't have ALL mm's covered by lenses.
This one is very interesting. I was very curious about it a while back, but we have very little in the way of high-end "try it out" camera stores where I am. If I'd had an opportunity to try one out first, (and seriously liked it) I might not have bought my 70-200 f/2.8 IS II, even though I'd have a hole between 55mm and 120mm. Not having actually tried it, I guess I have to sit down and shut up.