my "carry on" sized duffel is verifiably 3-5 inches larger than the maximum allowed dimensions, but i bring it on without a problem every time. i can't stand the idea of dragging some big and clunky thing -- which will surely break at the wheels eventually -- when i can just sling my bag and go.
More than once, I've been near the end of the boarding line, where they stop people from getting on the plane with their full-sized carry-ons because there's not enough room (which happens more and more frequently now). Every time, the backpack goes right through.
I've heard of people suggesting one of two methods of handling forced check-ins:
1) I use an app called MyGearVault that shows the value of the kit (also offers insurance). Checked bags are only insured to a maximum of 3,500. You can extend this to 5,000 for $50, but no more. "I do not want to check my bag because it is not covered by your insurance".
2) carry $50/$100 - if asked to check your bag, look behind you. "Hey, is someone willing to check their bag in place of me, here's some incentive".
I've never personally tried other method, but I've heard people claim success with either.