I use my Spirit 40 for travelling more than anything else. I'm a big guy (195cm/100kg) so I can 'only' fit three days' worth of clothes in it along with everything else I need for my trip. The separate pocket (along the back, loads from the side) works great for a laptop or a pile of maps. The side/top loading zipper takes a bit of getting used to, but once you figure it out, you can load your big stuff in the side, then zip it partway and jam smaller stuff in the top.
The suspension system (I've got the 42L 'large' version) is great. It would be nice to have a better way to tie back the hip belt, but all in all it's one of the best systems I've used in a pack this size--you can really load it up and it's still comfortable. Everything is adjustable, so you can move the straps around and balance the load between shoulders, chest and hips however you feel like doing. The 'lid' pocket is good for keys, wallet and phone, but not much else.
One thing it's NOT good for is being a third or half full--the pack is not set up to carry 2kg comfortably. It needs to have some stuff in it to balance correctly.
All in all, I'd rate it a 98/100. I never go too far from home without it. If you're a light packer, you can get away with nothing more than this bag on almost any trip, and you can walk practically forever with it unless it's full of lead bricks.
Incidentally, this bag fits fine in the overhead compartments on aircraft. It's a tight squeeze on commuter planes (Embraer RJ135, Saab 340, etc) if the pack is really full with a laptop in the back pocket, but it does fit. On 737s, A320s, etc. it slides right in with room to spare.