I've never been to the London one; I went to one of the mid-land ones, but that was a year ago.
I'd go if I weren't doing Fushicon (18th Nov), since money says 'no way you can do two events in a couple of weeks.
A lot of people go, and, to be fair, I have a balanced amount of friends saying 'they loved it' and 'there was nothing to do after two laps.'
I'd say go for the experience and see what you think afterwards...