I think that good customer service is what will keep a customer loyal and stay with a registrar, given that their prices are not too much higher than the other registrars. It's worth it to pay a little more to use a registrar that has good customer service/support.
However, if the registrar has good customer service but is much higher with their prices then most domainers will shop around and use other registrars. This being that a domainer will have a lot of domains and the higher prices to register and renew domains can really add up if you have a lot of them.
I have tried several and have 2 main registrars that I use, Namecheap & GoDaddy.
Namecheap is a little more for renewals and registrations than some but their customer support / service is the best I have had. I have used their support for both domains and hosting and they were awesome each time. I have never had anything but great support.
GoDaddy is pretty good as well from my own experience, as far as domains go. One of my hosting accounts is with them and I had a couple of issues with their hosting support and they really screwed up and basically destroyed one of my websites.
I hadn't done a back-up in a while (my fault) because I was in the process of changing some things and they destroyed the site and I had no way to restore it so I had to completely rebuild it from scratch and that cost me money.
I kept dealing with their phone support and they kept breaking one promise after another on things they said they would do and things they said they would do in a time frame. They also made several promises on different calls to them to call me back after dealing with them on the phone and each time they did not.
Their people who you talk to on the phone are really nice usually and that makes them seem like their support was better than it was in actuality.
As far as domains though, I have never had any issues and their service was always good.
I would feel comfortable recommending both, depending on what exactly a person was looking for. If it's hosting, I could not in good conscience recommend GoDaddy however and that's based on my experience with them. Plus their resources are some of the lowest with their hosting plans compared to other hosts so it makes their hosting seem like a good deal but the fine print (RAM, etc.) is what makes it not a great deal if you are hosting more than one website.
I think that most domainers would use a single registrar if their prices and support were both good but sometimes it's not possible due to some extreme variations in reg & renewal prices on some of the more expensive extensions.