Its not an L series lens in terms of build quality etc but IQ seems pretty good on the copy I have, no complaints on either 1D (almost permanently attached) or 5D.
I bought mine 2nd hand off e-bay so not sure if its had a good or bad life but all seems well with it. What I do notice is the IS doesnt seem to freeze the image in the viewfinder like it does on my 70/200 f4 L or make as much noise (less glass to move around perhaps?) where you really notice the image stabilising, I can hear the motors whirring but I'm not absolutely sure its doing its job, but it does seem to stabilise the image somewhat so I think its OK just not quite as definite as with the 70/200 IS (if that makes any sense).
Matt