I've got this (Swissgear Granada Rolling Case Nylon) -- fits under the seat on mainline planes unless completely stuffed and fits in the overhead of all mainline and connection carriers (at least in DL world).

Can easily fit 1-2 nights of stuff in the big pocket and easily handles my 15 inch Macbook Pro in the middle pocket. Looks professional as a rolling briefcase, I've never had any qualms rolling it into a meeting on a one-night trip and leaving it in the corner. I've had it for about 18 months and it's held up pretty well with multiple trips to Europe, Asia, and South America -- for longer trips I still take it along with my standard rollerboard and use it in place of a backpack.