Mainstream Paintball- the only other field close in the area, is a commercial field and is indoor.
Tunka isn't really a field, its an email group of boulder players to organize games. Usually the group plays on nearby national forest or the Miliken field. The people in the group are friendly, and the games are played with exceptional sportsmanship. There is always a chrono present and barrel plug/condom, goggle and other safety rules are enforced. Tunka players are generally friendly and willing to give advice about playing or equipment. Tunka is also the cheapest way to play paintball up in Boulder as it is BYOP and no extra costs whatsoever.
However, the group does have a few negatives in my opinion, although none of them are very serious. Because it is just a group and not a field, it is up to the players to provide enough air for themselves for the day. If you are running compressed air you'll need to get the tank filled beforehand or hope that someone brings a scuba tank. There is also the concern of no refs, but as everyone I've played with at Tunka was nothing but honorable I really can't complain. My biggest complaint with the Tunka group is that everyone seems to just want to run out and play before setting boundries or rules, making games really confusing.
However, the group is a good one to play with and lots of cheap paintball fun.