The cost per round trip, based on using 20-single-ride tickets, is $6.85. That's slightly more expensive than the gas on the van (based on the van getting 20 MPG), but far less stressful than driving, particularly in stop-and-go commute traffic on I-880/CA-237/US-101. The ACE ticket includes the shuttle bus ride; it's not a separate service that runs on an uncoordinated schedule.
One drawback is that there are only three ACE trains per day, and thus I must leave the office at either 4 PM or 5 PM or else I'll not be able to get home without an enormous roundabout set of bus rides.
I won't take the train every day (I wouldn't be able to go to BASFA on Monday, for instance, if I didn't drive, unless I rode the train home and then immediately drove back the way I came, which is silly), and since I don't plan to be in Fremont more than another three months, this isn't a permanent thing, but I'm happy to get back on the commuter train for a little while and save some miles on my poor abused minivan.
The ACE trains I've been riding seem about 2/3 full. When I see all of the people pouring off of an on to the train at Great America, I think of every one of those people in a single-occupancy vehicle and wish that the people who don't think trains are worth the investment could see it. Even if you never ride the train yourself, the service is making your drive through those areas the train serves less awful.