This is a very old lens dating back to the early days of Auto focus cameras (mid to late 80s). It was Sigmas first attempt at a wide angle lens with the Minolta A Mount and as such was quite successful. It's optics are very good, the A/F speed is fair, it does not suffer from any major optical defects or vignetting. It is however prone to lens flare as the front element protrudes out quite far. It also has a fixed "Perfect Hood" which is not really perfect as it is a bit too small to prevent flare. That hood also increases the length of the lens making it more difficult to fit into a bag. This may be the reason it was discontinued. Unlike some Sigma lenses, it works perfectly with the 7D and presumably with all other Minolta/Sony digital cameras.
This lens is very rare and is seldom up for sale. If you can find one then I would suggest that you buy it as they are fairly inexpensive due to being relatively unknown and a search on google reveals little. The optical quality will not disappoint, although at 21mm it is not really a wide angle by todays standards. Being one of the older lenses, it is built like a tank. Well worth $100 to $150 or more if in good condition.