As it happens, I just posted a trip report ("(Mostly) Amtrak ...") that includes a Groome bus segment. I thoroughly enjoyed the trip, and it beat the Amtrak bus six ways from Sunday. Hint: the Amtrak service is a bus, Groome is a coach. We don't usually make the distinction in this country, but there is a huge difference!

They collect the fares southbound in Petaluma, as well, which threw me.

I was actually trying to get from Rohnert Park to the San Jose train station, so took Groome to SFO, BART to Millbrae, and Caltrain to San Jose. Not sure I'd try this in the weekday evening rush hour, but it worked fine on a Sunday. And with a suitcase, even a rollaboard, I'm not sure I would try SamTrans, either.