Hate to spoil it, but I live slap bang in the middle of a 20 mph zone (LB of Bromley). Good intentions aside, it's not enforced and virtually every car drives 30-40 mph, using the area as a rat run to avoid the lights on the main road. Cars tend to bullet down the single lane in the middle of the road (because parked cars line the streets) like it's the barrel of a gun. There's some traffic calming of the variety that flings cyclists in and out of this line of fire. As a cyclist you need a firm will and love of primary, because you're periodically going to be facing an oncoming BMW driven at speed by ubiquitous mobile phone using driver.
Sorry, it sounds very negative. I think they're a great idea, but the implementation leaves a bit to be desired. Drivers, sadly, don't police themselves and having 20 mph where no one drives at 20 mph tends to void any benefit.