The trains in the South/South East of England are great. I grew up in Wales, and I have never before experienced such comprehensive train coverage. You can get pretty much anywhere in the South by train. It's quite pricey though -- more expensive than driving, even if you factor in insurance / tax / petrol etc.
Buses in Crawley are pretty average, similar to the town I grew up in. The city really was geared for driving, though.
London, OTOH, has an excellent bus network, which is still fairly cheap. Definitely cheaper than driving in any town, and cheaper than buses in any city I've visited in the UK. The underground is crowded and rather too inconvenient for me (takes forever just to get to the platform!), but it's still adequate. But the buses in London really do excel. You can get anywhere on a bus, for next to nothing.
EDIT: I voted based on my impression of public transport vs driving/other alternatives. I voted Good, since I'd generally prefer to take public transport than to drive, though of course this varies from place to place.