Boulder is unaffectionately called the People's Republic of Boulder by other Coloradans because it is such a far-left town. It's like the Berkley of CO. That being said, it's a beautiful place.
I dont live in Boulder but I have OCd there several times without incident. Their parks and open spaces are posed No Carry so you can't OC there. Of course, they cannot restrict CC in these areas. From what I've heard from people who have lived in Boulder, even though it's pretty far left, they are not all that anti-gun. Denver is certainly worse in that respect.