at SFO when I flew with United they had me put my bag in their measuring "box" to make sure it would meet their requirements. It was only a camelbak commander, but barely fit their rediculous small box. Just double check before you go, otherwise they would make you check it and that could cost you an extra $50-$100.
I used my Kifaru T1(with 26" stays) as a carry on when I flew from San Jose, CA to Denver and then from Denver back to San Jose with no issues. But I think I just got lucky. I think it depends on the Airport, airline, and mostly the security agents checking you. I knew I wouldn't have a problem in the San Jose airport because in all flights I've taken from there I have learned the security people there are lazy morons that hardly speak English. You would be amazed at the things I've made it through security with at that place.