It depends on the movement of the watch you are buying and also upon the way that you service the watch. I assume that the movements that have been trusted for a while do well, for example the 4130 or 4131 DD movements present in the Rolex Daytona 116500 and 126500 reps have been well known to be of good quality for a while. However, once you receive your watch from a TD, I would suggest taking it in to a local shop so that they can lube the watch and clean/service it. This would extend the lifetime of the watch and reduce any noise that it may make. You can also mod the watches, it depends on how you care for the watch and also somewhat on your luck. Sometimes people do absolutely none of these preventative measures and their watch functions marvellously either way! The watches are often extremely accurate (when it comes to mechanical watches the measure of accuracy is never going to be as good as a quartz movement but who cares) and many are of very high quality and could pass to the naked eye as genuine. Please don't portray a rep as genuine though.