I have had a custom Timbuk2 Laptop messenger bag for a little over two years now and it is great, unless you have to carry a bunch of stuff with you.
I had been waiting for the past 6 months to get an updated MBP 13" but when it was released with a C2D I just couldn't justify buying the 13". So I decided to get the 15". I am student so I carry my laptop everywhere I go but honestly my Timbuk2 bag probably would not have even been comfortable for touting around a 13" MBP + the usual stuff I take with me to school. Like all messenger bags, it just doesn't evenly distribute the load on your back and shoulders. Given this, I just bought a Tom Bihn brain bag. I haven't received it yet but everything I have read about Tom Bihn bags, including comments on this forum, have been fantastic.
Bottom line, unless you are only going to only carry a 13" around with you all day (i.e. with no other additional heavy items) I wouldn't get a messenger bag. But I know what it is like to have your heart set on a messenger bag. Unfortunately only experience taught me that they are a bad idea if you have to carry a somewhat substantial amount of gear around with you for extended periods of time.
By the way, if you want I am selling my Timbuk2 given that I just bought a brain bag. I paid $160 for the Timbuk2. It is a custom laptop messenger bag in large with black-gun metal-black color panels. It is in great shape. Let me know if you are interested.