Out of all the Kleiner restaurants I've been to (Opera, Gioco, Marche, Red Light, Carnivale) this is probably the most interesting in terms of design and history. According to Kleiner, Room 21's building used to be Al Capone's largest liquor warehouse and speakeasy. When renovating the building, they accidentally stumbled onto a secret passage and found a door leading to 21st street that said "Room 21". He tells the story better then I do:
If you made it to the end of the video, you saw the private dinning room that overlooks the kitchen. This was by far the coolest thing about the restaurant (and to be honest probably the coolest thing I've ever seen at a restaurant). However, they did tell us that it takes a lot of coin to eat in the private dinning room. A minimum of $1000 on the weekends and $500 on the weekdays.
In my eyes, the building and history kind of overshadow the food. Although everything we had was really good, nothing was amazing. However, with most Kleiner dinning experiences the food is only part of the package.
Besides the large dinning area, their is a large bar/lounge and a nice outdoor patio. I'm also a big fan of Eric Clapton and they must have played 3 or 4 Clapton songs, so in my mind the music was great. The service was prompt, but our waiter was kind of spacey and wasn't the best. However, after dinner we asked the manager for the "tour" and he spent about 5 minutes telling us about the history of the building and so on, which we all appreciated.
So all in all, Room 21 is a very cool and interesting place and Sloopin would highly recommend it. If you go, don't forget to ask the manager for the tour of the building, you won't regret it.