I am not naturally a bus person. I will use a bus when I have to, but I prefer trains to buses. There's something about not having to deal with traffic that makes traveling just so much nicer. I would say, and this really is a personal opinion, that by and large London and Washington are cities where you don't have to use the bus system unless you're feeling really lazy* because both cities are very walkable. I could pretty much avoid buses in London because I could either walk or I could get there via the Underground and the same can be said for DC by and large. Boston is something else though.
Boston as the T which is combination of light rail and subway largely depending on what line your using. I live about five minutes from a Green line T stop, but I've also been on the Red line (because that's the most convenient line out of Harvard) and I would say the T is largely on par with the Metro or the Underground. But unlike those two subway systems, which largely take care of any transportation around their respective cities, the T's main goal is to get people into downtown Boston and then out to major points of interest and/or suburbs. Unlike with the Underground or the Metro, there isn't a lot of intersecting of rail lines so if you want to get from point A to point B and you have to switch rail lines you're basically stuck going into downtown, switching lines**, and then heading out to where you actually want to go. That is if you're unwilling to use buses.
I think one of the reasons I don't like buses is that they seem more unreliable than trains. However because of the way the T works, buses and trains are basically on par for their unreliability in showing up on time.*** And because of the way the T is laid out half the time it's quicker to take a bus, even factoring in traffic, than taking the train because the bus will be more direct than the train. Since I've been here in Boston I would say I spend an equal amount of time on buses and trains where as in DC or London I spent more time on trains than I ever did on buses. The buses get more crowded more quickly than the trains, but in terms of time it makes more sense to try and find a direct route on a bus than taking a circuitous route on a train.
It's interesting though. I would say that the T is equally dysfunctional as the Underground and the Metro, so I feel right at home. The biggest difference for me is how often I take the bus when I really do prefer trains.
*Or it's a rail replacement service
**Or switching from one branch of the line to another (yay for the Green line having four different branches)
*** Because you have no idea when a train or a bus is going to show up. At least with the buses you know they're late and can reconcile that in your mind.