I felt it was time to replace my current seat bag after 200 miles of riding because it didn't seem like it was going to hold up much longer. That's what you get from the bargain bin at the LBS.
I noticed on the Timbuk2 website that they are making seat bags so the first thing I did was contact them and ask what size I would need to hold a multi-tool and a 29er tube. They replied with the large so I ordered one the other day and it finally came in.
As you can see it holds the tube and tool just fine. Also in there I have about 6 inches of chain, too, and there is just enough room for perhaps a gu pack and my keys. That is something that has always bothered me with the previous seat pack. I never felt that it would hold my keys securely and while I never had a problem the fact that they were just in the bag loosey, goosey just made me nervous. Especially if I ever got a flat and had to pull the tube out, my keys would drop right to the ground. I think I will feel a little more comfortable that the Timbuk2 has a key tether.
The bag looks pretty good under my saddle.
Goes nicely with the bike, too.
I think I am going to have to get one for the Monster Crosser.