I have a STM Small Alley bag, and have been very pleased. I was looking for a case that wasn't large and bulky, but still had good protection and storage - and this fit the bill. I also liked the attention to detail put into this case. Small things like keeping the rough side of velcro in places where it is unlikely to rub on the computer are nice touches.
One minor issue I have with the bag is that the compartment holding the macbook doesn't form a "weatherproof" seal. It is a fabric flap secured with velcro; so the two top side corners are slightly exposed. I haven't traveled with the case in the rain, so I can't report on it's effectiveness. Just something to keep in mind if that is important to you.
STM is a good brand I used to have an STM bag when I had a macbook, but now that I have a pro I use manhattan portage and incase. Both are brands that you would want to look at.
Incase makes awesome stuff for macbooks/macbook pros. They have sleeves, hard cases, and bags. I love my sleeve, and the hard case looks excellent as well.