I have a few in Moniker and Godaddy.
Moniker: The customer service horror stories are true. My one and only support ticket took weeks to close- I presently have an email in pertaining to an aftermarket industry software product that needs their assistance with setup. Days and days and days have gone by. The one time I called, the lady I talked to made me think I had accidentally phoned in to Auschwitz.
But, the interface is nice, clean and simple to use. Hugely superior to godaddy, plus, they seem to have a presence on these forums, so if I ever get really mad and start a "Moniker Customer Service Sucks" thread, at least I know Bari will pop in and let me know she can fix it. I sure wish that kind of service were available on the front end...
Godaddy: Fantastic customer service.
Like, great. You can tell the corporate culture there emphasizes this, unlike other businesses who view customer service as a grudging obligation, where they just say whatever they can to get you off the phone, then laugh at you after they've hung up.
Downside: the interface sucks ass, the upselling is intolerable and by all reports Bob Parsons is a person of questionable character (which may or may not be true, but my poker-player intuitive soul read is that he probably is)
If godaddy were to keep their main site to appeal to the domaining 'masses' yet develop some sort of lite version for serious domainers who don't want the upselling, tacky ads and desire a more streamlined name management system- maybe like a godaddyPRO.com or godaddy.pro- they just may crush their competition.