This is easily one of the coolest things to do. You have to text the phone number to set up a reservation. They will prepare a table for you at the designated time. When you get to the outside bar (The Commonwealth), you will send another text message. You will receive instructions on how to get in.
The atmosphere is very near what I would expect of a true speak easy. The woman who acted as the hostess was very friendly. The bartender (Dan) was great. He helped my mom and I choose our drinks when we weren't sure what to order. My brother had an old-fashioned. All the drinks were good (although they are a little pricey, as mentioned in many other reviews).
Our only complaint from our visit was that at the end of the night, we waited about 45 minutes after receiving our first and only drinks. The waitress finally came back around and we asked for our check. 35 minutes later, still no check, so we broke the rules and walked up to the bar to pay. She seemed confused as to why we came up there to pay. To make matters worse, she has just been hanging out at the bar chatting, not preparing other tickets or serving drinks.
Overall a good experience! I'd definitely go back again.