I've visited the Cup for both breakfast and lunch, though never dinner. I love the decor and atmosphere at the Cup. It's open and airy and comfortable, and it feels a touch European somehow. Perhaps because the Cup is in a historic building, rather than in one of Tucson's many strip malls.
Breakfasts are very good at the Cup. They have vegan french toast, and you can substitute tofu for eggs in other dishes. They have excellent coffee, as well.
The Cup has several vegetarian and vegan options on the lunch menu. I'm a fan of the Queer Steer (veggie burger) and the Volatile Veggie sandwich.
The dinner menu has fewer vegan dishes but still looks very promising. However, as I've never ventured there for dinner, I can't actually testify to how good the dishes are. The "Katmhandu" (vegetables in a banana curry, served with coconut quinoa & garnished with banana chips & black sesame seeds) sounds decadent.
The Cup also offers a variety of specialty coffee drinks and alcohol-based beverages. There's Club Congress (a popular nightspot) and a bar in the same building (which also is home to the Hotel Congress). I've never tried any of their very tempting mixed drinks, mostly because I feel a little awkward having booze with lunch (or breakfast!).
The dessert case at the Cup looks divine, but sadly they don't have any vegan desserts (as of the last time I was there). The lack of vegan goodies drops their rating from a 10 to a 9, in my book.
Finally, a neat little historic fact: John Dillinger was captured here in 1934 after evading police across the country. Also, the hotel is supposedly haunted.