My business is largely split between Enom and Registerfly. They both have occassional problems, but I've had less there than anywhere else I've used. Both offer a good price with lots of features.
I do like registerfly's new transfer where you just enter the email of the new owner and they accept the domain from the email. It kind of sucked before that, since they required the password of the account to push to.
The new domain marketplace was kind of innovative and beat the waitlist to the punch. I've bought and sold domains there, but the catch is they have to be registered with RF. You have to watch it closely though or you could end up selling too cheap close to regsitration due dates. (buy prices drop daily based on pro-rated time left on registration).
RF problems have been much fewer and far between lately. I think they've pretty much worked all the bugs out of the older account control panel scripts.