I think NWA's website is one of the easiest to use. I like their short-cuts in the main page. However, it would be nice if NW gives people the ability to import reservations made outside of NWA.com into their profile, such that they can change seats, request meals, etc.. CO has this feature.
I find CO's website rather annoying and not well-structured, not to mention their numberous down times!
- Pat