Sigma's primes are not without their problems though. The 50mm in particular suffers from focus shift when stopping down, the issue is worse on Canon cameras because the autofocus has the more sensitive f/2.8 centre point as opposed to other makes on f/5.6. When you focus with the lens wide open at say F/2.8 the focus is at one point, when the shutter is released the aperture snaps down, and the point of focus moves, in many cases ruining the shot.
This has been the issue many users have reported since the lens was released. On the other hand Canon's own f/1.4 although optically good suffers reliability issues, and should really be replaced. The 80mm f/1.8 is a very good lens, cheap too, it's difficult to see why anyone would pay a lot more money for a one stop advantage (especially if it suffers focus shift).