Ive done London a couple of times and loved it.
Cons
The traffic is a mare and having to rack Sat if you are racing Sun for an Olympic is lunacy.
The swim is busy ish and its a deep water start (actually this helps spread the field) Try IM France for some proper punch ups.
The bike course is great if you go all the way to Westminster but dull and narrow on the other route with some safety issues caused by the number of cyclists and novices on the course
The run has an annoying ramp at the end
Its expensive for an Olympic , though others are catching up now that Human Race at Dorney and Windsor have caught on to what they can charge.
Pros
Its a great weekend with all the hoopla of the expo and great for becoming part of the bigger triathlon gig.
The bike course is flat and fast
Lots of other novices do it so you are unlikely to find yourself lonely and demoralised at the back
Support on the run probably the best in the UK. Windsor is quite close though.
Id say it one you should do at least once as a Triathlete , it is a bit of a marmite event people tend to love it or hate it. Personally I thought it was great and only havent done it a third time because of the long stuff not fitting in with it. My view is biased by the fact that Tim Don was in Transition warming up when I came into t2 one year and told me to hurry up !