There is an $8.95 entry fee for adults, but a free coupon can be found on their website. Beware of the easily-confused cashier who was like the guy from "Memento" that can't remember what just happened.
Although some sites say the museum is on the fifth floor of the parking structure, the entrance is not. To get to the collection, you must be in the Imperial Palace hotel - not the parking garage.
The collection during my visit was mostly Rolls Royces. It felt like Cadillac was the second most common name. Big cars by Auburn, Duesenberg, or Packard weren't there on my visit. There are 100+ cars so allow 30-minutes or more to see all of the vehicles.
The showrooms were run-down (like everything else at the Imperial Palace) with air conditioning that rattled or didn't work.