I have been using them for nearly 10 years now, first they were just called "1St" but then in 2006 when their parent was bought out by Godaddy it became godaddysitebuilder - which has since changed to what we know as First Domain today! They are an excellent registrar/hosting provider with fast customer support (which isn't always great!).
It's very easy registering domains through fdns or via our hosting panel at Hostgator & GoDaddy etc., you can also use your own nameservers if that fits better into how things work elsewhere e. g. Namecheap / CloudFlare services, DNS settings etc..