I also have, and heartily recommend, the Tom Bihn Pilot. For me, it's big enough to do an overnight business trip (with laptop) as my sole bag (well, usually with a smaller purse-like object, and note that I'm not a suit wearer, although I will bring a second pair of shoes), and there's no problem fitting it under any seat while not taking up all the room under the seat.
It's also otherwise quite functional when I'm not traveling and need to schlep a laptop and lots of stuff. Especially in the halcyon fabric, I like that the bag itself is very lightweight. Adding the Absolute Strap is good for heavier loads.