It's OK. They have nice travel busses that they use most of the time but any day they can run out of them and I can unexpectedly get stuck with an uncomfortable regular bus. I stopped taking Transbay most of the time and switched back to Emery-Go-Round and BART after a few months of taking it.
This was with being one of the last stops before SF and having a good onramp to get onto the freeway. I tried taking it from the east oakland hills when I lived there and it was completely unpalatable.
I take AC Transit every day (for the last 5 years) and if you get on at the last stop it's standing room only and terrible where busses sometimes drive by you. But, from my house, I have a seat every single morning no question and in the evening, as long as I'm ~10 minutes early to the stop in SF I get a seat.
I ride the F, which is a "normal" bus, but it's 10x better than BART for comfort, temperature, smell, etc. The trip takes me 40-50 minutes on a normal day.
This was with being one of the last stops before SF and having a good onramp to get onto the freeway. I tried taking it from the east oakland hills when I lived there and it was completely unpalatable.