For buses it is very effective, since they stop every few blocks, whether the stop signal is red or not. It also makes for a huge saving on brake repair and replacement, which you can imagine is a big expense for buses.

Also city buses usually drive under 200 miles a day and then sit overnight so you can have relatively small batteries. All in all, it is hard to think of a type of vehicle where battery power makes more economic sense, and gives you more advantages over ICE.