I don't mind a progress bar that doesn't move perfectly smoothly, as long as it's MOVING and not stuck in one place for a <b>very</b> long time (making you ask yourself, "Did it crash or is it still good??").  My biggest peeve, though, is when it's accompanied by a time remaining estimate that continues to GROW before it diminishes.  So especially in the working world, when you tell your boss a render or upload of an important project should take X and then it turns out to take much longer, that's not good!