I've been eyeing the Think Tank Street Walker backpacks for some time, but I wasn't sure which size to get. Tenba also makes a decent backpack, but after ordering one to play with, I found it to be more rugged than I needed. Half the depth of the bag was in the padding and straps. At the moment, I need a bag good for airline travel that'll carry my larger lenses. So the decision then was between the two Think Tank models. While I don't think I could go wrong either way, I'll probably end up keeping the smaller version. The larger pack is close to the size of my largest bag - so it would be rather duplicative. The smaller version offers sufficient room without allowing me to take all my gear - which isn't the goal. Photography isn't typically the primary goal of trips this bag will make. Yet there's still plenty of room for my D300, 24-70, 70-200, 50mm lenses, and a flash. If I can make a great shot with that list, I need a new hobby.
The bag is light weight and should fit under an airline seat with room left over for my feet. It'll be easy to lug around in the airport, and it doesn't scream 'photo bag, steal me!'