I've taken my hiking backpack as my carryon plenty of times -- all over the world and frequently on NW. It isn't an external frame and I compress it down to a reasonably sized pack with the straps on the top and the sides. Basically, do a reality check once you've packed it at home -- can you fit it in the overhead easily? Is it any mroe ridiculous than what other people usually take on the plane? And be prepared to gate check it just in case, though I've never had to.

peace,
~Ben~