This is a go-to restaurant for when family are visiting me in DC. It has a fun environment and interesting menu. The menu changes so sometimes I've been happy with my food and sometimes not. I remember during one of my first visits, I had the vegan fish and chips bucket, which was a ridiculously fun and indulgent meal. During a recent visit I ordered the "cauliflower steaks" and felt completely ripped off after paying something like $15 for little more than a few pieces of grilled cauliflower. During that visit, everyone at my table felt bad for me because it was clear there weren't enough calories on my plate. The most recent time I ate at Founding Farmers, I was much more careful with my ordering and asked the server for a recommendation. He said vegetarians seem to like the meatless mushroom swiss reuben, so I ordered that and did actually get enough calories! It wasn't a very complex meal in terms of flavor, but it did satisfy (I seem to remember it wasn't vegan).
Overall, Founding Farmers can be a good place for a mixed meat-eating/vegetarian group, and can be especially good when family and friends are visiting. The food can be hit or miss though, so be careful when ordering!