I've taken them twice from Marin to Oakland airport and back.
Each time they've been on time picking up and dropping off.
Each time, I've been the only passenger or there's been one other person.
Each time the driver has been pleasant, and the van clean.
Once it took them about a half hour to pick me up at the airport because they couldn't find me, but that's not long to wait, and I've been happy with the service I received.
At least in my experience, it's a good way to get from Marin to OAK, and ends up costing the same as a taxi to the Sonoma airporter (which doesn't run every half hour like the marin airporter to SFO).