The interesting thing about all this is that there are precious few true (as in Curitiba) BRTs in the US, either existing or proposed. True BRT is a separated facility, much like light rail, but with buses rather than trains as rolling stock.

Something like the LA Orange and Silver Lines is BRT. It mostly runs on a separate right of way and has station-like bus stops, similar to light rail. A limited-stop or express bus on the street, with or without fancy "stations" for bus stops, signal preemption, and exclusive bus lanes in bottleneck spots, is not BRT. It's enhanced bus service. That might be useful, and in cases where there's no practical way to build a separate bus lane it may be all that can be done. As Bob notes, the effect on car traffic needs to be taken into account; while some effect on car traffic can often be accepted if the buses are actually helped, it's a balancing act and too much reduction of car capacity results in a political liability among other bad things.

SF has long had problems with the bus lanes on Mission Street. They help the buses, a little, but are often violated by drivers resulting in bus delays and accidents. At least, they've been there long enough that most drivers avoid Mission Street - just too much of a hassle. It'll be interesting to see what happens with Market, though, after their big new project to exclude private cars from it east of Van Ness really gets going (there's hope that the project will improve transit speeds on Market Street back into double digits).