I love this place- lots of vegetarian options (my favorites include the Apple Jack sandwich with red onions, monterey jack cheese, and apple slices, and the veggie version of the frito chili pie)
Their daily specials are almost always veggie-friendly as well. They also have some pretty fabulous coffee, and they host a lot of poetry readings, concerts, and meeting for local clubs like 'stitch and bitch'. Great food at seriously good prices, and very community oriented. A very nice place to chill out.
I can't give them unqualified praise, though, and I should mention a couple of things. One of my friends says it feels uncomfortable, like you have to know the 'rules' before you go or all of the hipsters will give you attitude... So, to spare you all that fate, here are the rules:
1) As mentioned in the little blurb, they ONLY take cash, no credit or debit cards, no checks. This is the only really frustrating thing and frankly, this is the reason that the Red Cup is only a special occasion place for me. Everyone uses debit cards! (p.s. there is a 7-11 across the street w/ a 'no surcharge' ATM.)
2) Parking. Pay attention to those little signs that warn you away from the dentist's office- they will tow you! There is another parking lot off of Classen (a block S of the Red Cup).
3) The hours. They are open only until 3:30pm mon-wed, but until 9 or 10 the rest of the week, and 12:05-4:37 on Sundays... (well, not really, but check the hours before you go)
Keep that in mind and you are in for a treat.