It was down for 4 hours that I know of - nothing from nominet (of course).
You can use it to get info about domains on your tag. However it is only updated once per day. They also offer a SOAP Api which I use as well but that doesn't give you details on all your domains.
Therefore you have the silly situation where you can view a list of your domains with expiry dates, renew one, and when you next load your list it won't show you the new expiry date until the next day. So really it's not too much use and probably why registrars have issues as you cannot trust the information to be *current*.
Still handy for a bulk overview of your account though. All they need to do is provide a list of expiry dates in the soap api (which is current) for all domains and it would be FAR more useful.