This is a great little place behind the Library of Congress. I've heard it can get very loud later in the evening, but when I've been for after-work drinks or dinner, it hasn't been bad. The dining room downstairs is an interesting mix of very modern gleaming silver tables and chairs, gray walls, lots of flickering candlelight, balanced by a dark wood bar and partial brick wall. Not cozy, but very sleek. There's an upstairs more lounge-like room which seems to be booked often for private parties.
As a disclaimer about the food, let me just say this is not a place to go if you are looking for a hearty meal. The general theme here is small tastings, whether a charcuterie plate (different types of thinly sliced meats like prosciutto) or a cheese tasting. My first (and last) cheese plate here consisted of 3 very small chunks of cheese served with crackers on an exceedingly long white plate. At $11, I don't recommend. Maybe if you are just having a glass of wine at the bar and want something to nibble on. Similarly, the charcuterie plate with Spanish ham and buttery foccacia bread had enough to give each person in our party (3 of us) a taste.
Beyond the cheese and meat plates, Sonoma's menu is split into "firsts" and "seconds" which are roughly like appetizers and entrées. Most of the appetizers can be doubled in portion to make it more entrée-sized. My first time eating here I ordered two regular sized "firsts" and that made a good sized meal. The menu here changes seasonally, which is both good and bad. In early fall I had a fabulous shrimp gnocchi, which is not on the menu right now. I also had the Amish-pulled chicken with penne pasta, which was a bit on the salty side because of the cheese, but I would order it again. The price range for "firsts" is $9-$12. Seconds run in the $19-$22 range.
A popular and less expensive dining option are the personal-sized pizzas, where you can choose the sauce and toppings. Prices are similar to the first courses, $9-$12, depending on how many toppings you want. They offer many different types of cheeses and some slightly more exotic toppings like wild boar sausage. You won't walk away with any leftovers, but the food is very good.
This may sound strange, but I keep coming back here despite the fact that I have yet to have a great, or even worth ordering again, glass of wine. I enjoy trying different types of wine, and certainly this is a great place to do that, but so far a memorable wine has yet to be imbibed here. There is a sommelier, although I've never received any recommendations from him. I follow what I like, with suggestions from the servers. Probably not the best place for a serious wine connoisseur, but a good place for trying out different things, both of the wine and food varieties.