The "rumours/information/comments' I've heard is it boosts your career prospects with respect to gaining a pupillage or it's the thing to do if you're going down the Bar route - it might be the latter. And that it's not particularly useful, like LLM programmes in general, if you plan on being a (commercial) solicitor. Obviously it's one of the most prestigious PG courses in the world at an equally prestigious institution. I'd wait for the more authoritative figures to see this thread before you take that as gospel though.
Out of curiousity, which university are you doing your first degree at?