The Sun (Mountain Sun on Pearl and Southern Sun off Table Mesa) was a regular stop when I lived in Boulder; I was vegetarian then and, given a customizable veggie burger and several other options, had plenty of choices. I recently went back to visit Boulder and stopped in (OK, three times). I'm vegan now and it was still a great place.
The decor is fun: Boulder hippie, Grateful Dead-inspired stuff. Their beer selection is phenomenal (though they ran out of my favorite Rouge de Brouge just before I arrived). There is often a wait, sometimes a long one, but it's worth it.
One of the things I really like about the Sun is that all employees cycle through all jobs: cook, wait staff, kitchen help. They all tip-share. This means that everyone there knows exactly what's in everything on the menu. I was quickly told that the veggie burger is vegetarian, not vegan. While that was disappointing, the remaining food options were great. They were, however, only three: a portabella sandwich, a tempeh sandwich, or the veggie burrito (all minus cheese). I'm not a big mushroom or tempeh fan, so I had the burrito. Two days in a row. Since I didn't get cheese, I smothered it in salsa. Fantastic!
They also have daily specials. On my first (of three) visits this trip, they had something called the Yellow Submarine: a vegan curry. It was simply the best curry I have ever tried. I asked for the recipe, offered to pay for it, but was given it complimentary.
Yes, vegan choices are limited. Vegetarians have more choices. Nevertheless, the food that you can have here is delicious. Don't pass it by!Pros: excellent food, knowledgeable, helpful staff, incredible beer
Cons: often a long wait, limited vegan options, can be noisy