Personally I've owned six Canon lenses (still have four of them) and they have all been excellent. There have been a couple of Canon lenses (100-400 IS and the 400 f4 DO) that seemed to have quality control issues in the early days, but they now seem to be fine. I guess as with anything you could get unlucky and get a dodgy one, but if you did it would soon be replaced or re-calibrated.
I have seen it suggested that the reason that you hear of more issues with Canon lenses than other manufacturers is that they are selling significantly more units. I guess the safest way to do it (if you make the change), would be to go to a real shop so you can test the gear before buying. Or at least buy from a local mail order company with a good returns policy. Thankfully it's quite easy to test that a lens is sharp and that it is not front/back focusing.
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