I used register.com back in the late 90's...then godaddy for the last 6 or 7 years. At godaddy, I had a rep who was ok. But he was slow, had a shitload of rules and they're just not adult-friendly.
I recently switched to Moniker. They've never lost a domain (not many registrars can honestly say that), they're adult-friendly (ever seen godaddy at a show?) and their customer service is truly incredible. I've got my guy (Chris Leggatt) on ICQ all day, his cell phone or a 24/7 customer service number at night. Their customer service not only answers quickly, they're also no-bullshit easy and effective. I'm really...really happy.
