I visited the Letterman supercharger on Sunday. There is a guard stationed on the way in. When I told him I was going to the supercharger, he said that I should be sure to have my card or fob, as there is no cell phone coverage in the garage. He also said that there are no restrooms available as Sessions gastropub was closed. Also, no charge for parking that day.
Here's my video review of the supercharger: