I got this lens to use with a 5D about a week ago. Haven't used it a massive amount yet, but first impressions are good. The af seems pretty quick but like the reviews say it is a bit noisier than usm.
On full frame it is really wide, but on a crop camera like your 550d the focal range may be a bit limiting - it won't be that wide.
The lens is old, but it does feel great and from first impressions it looks good wide open - which is why I bought this rather than the newer 17-40L. I'll try and post some pics at some point in the near future.
One thing to watch out for is that the lens is old and has now been discontiued for about 14 years I think so parts are hard to come by, so only get it if it is in a+ condition.