I've done the training too and it was great, very practical and really useful. I use the Boris bikes and I certainly don't manage to go very fast on them, but it's great to be able to zip around on them at will.
They are slow but I suspect that's a deliberate choice of gearing. It makes for a safer ride for those unfamiliar with cycling in general and nervous of london traffic. I've actually quite enjoyed them, though my own bike is a very different beast.