Just my two cents here ...
Performance @ Bailey's is my LBS. I've tried quite a few local shops and the lead mechanic Tom is the only one I'll let touch any of my bikes. Also, when I go in I usually know what I'm looking for and the service has been great. I appreciate the fact that, if I do ask for a recommendation, they'll tell me what's best for me and not what's most expensive. The Performance @ Tyson's is good too and their mechanic is also top notch.
Revolution @ Clarendon is okay but, to me, it's way too expensive ... service and parts in particular. They're friendly, though. I personally prefer the one in Georgetown but that's from a service perspective.
REI is another great shop and I like it quite a bit ... they're fast with service and reasonable. A little pricy but not too bad. Plus, Phil knows his stuff.
Bicycle Pro Shop in DC is another great shop. It's small but they've got a ton of bikes in there and good mechanics.
Finally, I will absolutely, positively recommend that people AVOID Big Wheel Bikes in Arlington. They are the absolute worst shop in the area. I've given them several tries over the years and each time I walk away asking myself why. The Alexandria store is okay but has a limited selection of accessories. It's the same in Arlington but the people are either completely unhelpful or just downright rude.
Sorry for the lengthy reply but someone had just asked me this question so it was fresh in my mind! Plus, I'd rather be typing this than actually working.