Preface: I've never been to Benevolence for dinner or for Sunday brunch, so my review is solely based on the lunch options.
The food at Benevolence is decent but not particularly exciting for vegans. I've had some good dairy-free soups and the homemade bread is wonderful, but the other vegan lunch options are pretty uninspiring (and you have to make sure to mention that you're vegan when ordering - I once ordered a vegetable salad which came with sliced hard boiled eggs on top, even though eggs were not mentioned anywhere on the menu). I've had pretty mixed experiences with the baked goods at Benevolence, which tend to be a little closer to "health food" than I normally like in a dessert (but some of them are still quite good).
Pros: The ambience at Benevolence is fun - you end up eating in the middle of an environmentalist/leftist bookstore and the food isn't too expensive.
Cons: The seating is limited and most of the tables are fairly large, so you're likely to end up sitting with strangers (which is good if you like meeting people, but bad if you're socially awkward).