it's mostly down to personal taste and needs
get one that you like the look of, no point getting a do it all bag that you wouldn't be seen dead carrying around
or spending far too much money on something that does everything, when all you want it for is carrying the laptop and the power adapter, which is just as bad as spending too little and getting something that doesn't cater for your every need.
the one I use is made by case logic, looks like a normal backpack, but by heck it's got compartments everywhere, and is very spacious, enough to carry 2 laptops, plus the power adapters for both, and still have plenty of space for document wallets/folders, a cd case, mobile phone, ipod etc
although, when fully loaded with 2 laptops.... it ain't comfortable to carry
kinda of like the
Brenthaven Expandable Trek Backpack for up to 15,4" MacBook Pro - Black/Grey
I also use the Huzzk Sleevie GT just to make sure
I tend to avoid the messenger bag types, as most of them scream notebook! steal me!
that is of course my personal opinion