Rode it in 2015. Fantastic weather but not too hot. It was the year after the floods. The success of the day does depend on the weather. I still believe too many people ride it and so when you get to the Surrey hills the climbs are crowded particularly Leith Hill which is narrow. Also there have been unfortunate incidents which have created the route to changed and delays (last year I believe was bad for the later starters).
That said riding on closed roads in London has to be experienced so do it. I'm doing a Velo South this year.