Here's the perspective from inside one (jump to about 1:15 into the video):
• Video Link
If you watch all the way to the end, it actually transitions to a tunnel through a hillside. Kind of neat.
They're not really that much slower, and given that they don't have to stop or mix with traffic at all, they're probably faster in many cases. They also work well for narrower streets as you find in many downtown areas.
It's also possible, in theory, to integrate stations with new building developments with some coordination with developers.