Ok so back from our trip over XMAS. Booked the Connection service on both inbound and outbound on the ATL-SFO legs. Got P-Car transfer both times. Took us up the back stairs. Also booked the Departure service on the way out and while the agent was nice, it wasn't worth it. SFO doesn't use the P-Cars due to the gates being spaced quite closely together. There isn't a security fast-track at SFO either, although the agent did attempt (and succeed) in getting us into the PreChek line. We had to return the car, hoof it from rental car return w/ all our bags, two car seats and two kids and do check-in alone. The agent met us at the gate. In VIP's defense, we booked the night of Christmas for a flight on the 26th. To be clear, not complaining. But just wouldn't pay for departure again, particularly with an early AM flight. The service did help us make a super tight connection on the outbound, although he basically put our stuff on the plane and then disappeared rather abruptly.

Some other points:

1.) ATL agents told me P-Car transfer is pretty much standard. In fact, over the holidays (when one would think they would be swamped) they were so dead that the first agent told us she spent more of her day "Surprising and Delighting" high tier medallion members. They will drive you to the SkyClub of your choice and are VERY proactive about flight delays.
2.) JFK is apparently the next most common for the car transfer.
3.) Yes the car transfer is the thing for me (I hate walking through airports with two kids and all their crap).