I went with the blue sportster bag from the apple store. What I like about it is that it is very light, yet is well padded. It does not have room to carry much else, but with the lightness of the Powerbook, I didn't want a case that weighed ten pounds. I like the rubbery grip at the bottom that makes me feel confident carrying it from the bottom without the strap. The strap is very nice, with the same grippy stuff at the top, so it does not slide off my shoulder.
The other cases are great if you want to carry a bunch of stuff, but this case is great if you just want to carry the computer and a cellphone and sunglasses. There will definitely be people who don't like it though. When i carry my poweradapter in the zip pocket in the back, there is a little lump in the bag there. I don't mind, but there are people who would.
One last thing. I have a few backpack/laptop things from going to a few Cisco conferences, the padded case inside them is way to small for a ti book, but the sportster is the perfect case to put inside of the backpack, when I do want to carry some gear with me. The flexibility is great.