Dave-
I used Passport when I had the M shipped from Spartenburg to NY back in Jan '03. I was mostly happy with them, but I learned a couple things along the way.
First, it's not uncommon on long hauls for vehicles to be moved out of one trailer, and into another-- some times in a warehouse for the night, I was told. You can request that there be "no transfers", and they will honor the request. The other thing, is that you want your car to go in first-- preferrable on top (so nothing could fall on it). Cars that are not in first have to be jockeyed in/out of the trailer so the one ahead of it can be backed out. That's where damage can occur. In fact, I know a guy who had his antique Rolls almost fall out of the truck while it was being "jockeyed"--- only to be saved by the military-strength battery box!!
Lastly, are the tie-down methods. You want the carrier to use tie-downs over the tires only--- and nothing through the wheels. And for obvious reasons, nothing through the suspension (but you know that). Most will also use some kind of protectant over the car, and on the seats-- lightweight plastic drop-cloth kind of thing.
Good luck, and have fun in Sin City....