I use both GoDaddy and NameCheap quite actively with about 70% of domains parked at GoDaddy.
While as Blue said, I love NameCheap CLEAN check-out process (I wish GD had the same!), GoDaddy has really won me over at the start of my biz relationship with them by fixing a few billing issues when they didn't have to. I only had a couple of domains at the time and it showed me how bad they want to retain ALL clients.
As for NameCheap, the fact that I have never had to contact their support speaks highly of the level of their service. And yet its also the only factor that prevents me from knowing exactly what their service is like. In other words, I won't know until I get to contact them when I have an issue. 😀