I've had two powdercoated, the oldest being 5 years ago, and both are still looking good. Like you said, it all depends on the prep work, so its a good idea to pay a little extra to get that done properly. Make sure the shop you use gets all the old finish off, gives the bare frame a good wipedown with some pre-coat prep solution (usually a mild phosphoric acid solution) before the apply paint or powder. One other item to evaluate is if the shop does a lot of 'odd' pieces - car parts, furniture, metal railings - so I know they had experience with strange angles and tight areas that are tough to do with powdercoat (such as around the bottom bracket). Production line shops that do the same parts over-and-over are less likely to know what they are doing when powdercoating the tight areas.