Whatever diet you can come up with, Roots can probably accomodate. Plenty of vegan, gluten-free, organic, free-trade, raw, etc. They have a great vitamin/supplement section, and a lot of hard-to-finds drinks as well (alcoholic and non). It's not as big as a chain grocery store, but is quite substantial.
I tend to buy fruits, vegetables, and basic groceries locally and drive over to Roots every now and then for specialty items. The prices for the soft drinks/packaged foods/snacks I buy are as good or better than anywhere else. I suspect their organic vegetables cost more, but still in line with Whole Foods or similar stores. They have a lot of bulk grains and legumes.
Roots is on Georgia Avenue near the exit for the ICC. That being said, it is still a ways to go if you don't live in Olney. I wish Roots would open up a store in Gaithersburg or Rockville (their pet food offshoot, Bark, has done both). Other than that, my only criticism of Roots is that it's not entirely vegetarian/vegan (apparently it once was). It's a nice-looking, friendly place with great hard-to-find goods. I don't know of any better such stores in the area.