I think they're "safe"--I don't think they have SIP though. Our rental van that we had for a few days after DH totalled his car had an integrated child seat, and DD (5 yrs) was dying to try it out...so we let her use it on a trip to and from preschool...it wasn't very comfortable for her, either, she complained most of the time she was in it!
Also, if it's built in and something happened...ie, your car broke down so someone had to pick you up, you wouldn't have a carseat for your child.
Personally, I'd use them occasionally (ie, extra child not usually a passenger in my car, or for a carpool), but for my own kids, I'd want my own seats:twocents: