I've done it the last 2 years, it's very flat so ideal prep for Outlaw. Nice atmosphere, roads aren't too busy and it's a pretty run.Being late spring you just don't know with water temp, but that's the same everywhere.
Also be aware that you won't get to ride up to the Houses of Parliament unless you start in one of the very early waves on Sunday - you'll get turned round before Tower Bridge which means that all you get to see on the bike course is urban highway. And other parts of the bike course around Excel are very congested, as is the whole run. On the plus side, it is a great atmosphere - it's what got me hooked back in 2010. Expensive though, as has been said.