I live on U Street and for the last year I’ve been a member at Mint. Its expensive at $110 but I love it and think they have the friendliest and most responsible instructors of any gym I’ve ever belonged to. The classes are great and I go nearly every day. So its worth it to me.
Before that I went to the Results on U Street (pre Vida) and then Washington Sports Club in Dupont.
Most gyms let you do a multi day trial and then give you a deal after the end of the trial period. I would recommend that with any gym you are considering. Also, what I learned with my most recent experience signing up for Mint is that DC has a cancellation period that protects consumers, so if you decide you hate it within a week (or 10 days I can’t remember) you can always cancel it.