The classic example is the "Fail Whale" on twitter. But I'm interested in what people's thoughts are on this.
closed as not constructive by MDMarra, Shane Madden, Ward, Michael Hampton♦, Scott Pack Sep 23 '12 at 0:55
As it currently stands, this question is not a good fit for our Q&A format. We expect answers to be supported by facts, references, or expertise, but this question will likely solicit debate, arguments, polling, or extended discussion. If you feel that this question can be improved and possibly reopened, visit the help center for guidance.If this question can be reworded to fit the rules in the help center, please edit the question.
WRT a 503 and "best user experience" ... it's rather ironic question isn't it? But for best experience for the user, information is king.
So, make sure to include some information for them about where they can find further info -- a link to your status page for example. And then of course, make sure that information about what is happening, and when it will be fixed.
As far as the "fail whale" being a classic ... that's because it was seen so often.
As far as best user experience, I suppose the whale is entertaining. I have seen many funny 404 and 503 pages. Entertainment and information are the key in error reporting, imho.