I used three messenger bags when in College. Overall they were very useful, sturdy, and somewhat trendy I suppose. I ended up selling them and using a good backpack. Backpacks are more comfortable more efficient in their use of space and have easier access. They also have different compartments that make sense, you don't have to open a large flap to access a pen or your cell phone and you can stand up a backpack on the floor (recent messenger designs have improved on this). Messengers can be swiveled around the torso to open without setting down, which is handy sometimes.
I used a medium sized Timbuk2, an XL Timbuk2, (both were the "classic") and a Crumpler Cashmere Blazer. The Crumpler was extremely well made and used material that was very robust. I really liked it and ended up selling it for more than I paid for it. I used it for three years and it still looked new. The problem with this one was room. It was the largest model they made at the time (now discontinued) but due to the many internal pockets and padded sections, I could only fit about 7 inches of books inside along with my laptop, mouse, power cord, mouse pad, 10-key. It was very tight. I had to put the books next to each other lengthways to work. The Med. Timbuk 2 was just too small for anything but lunch and did not fit my laptop period (I had this before college and tried it for a while, ended up using for gym stuff and diaper bag later).
The XL Timbuk2 was fucking huge, no really it fits like three basketballs inside. It had enough room for everything (nine inches of books, lunch, water bottles, computer stuff). The problem is that it is all in one compartment and all that weight on one shoulder makes it uncomfortable to bear. No external pockets means that I had to open the velcro flaps just to dig through the huge interior space to find what I needed.
In the end the backpack I still use daily (a Dakine 101) and it is the best solution for me. A small pocket underneath the top carrying handle holds mp3 player, headphones, cellphone, keys, can of skoal, pens, flashlight, gum, small 3x5 notebook etc. 2 side mesh pockets allow grabbing a drink while wearing, separate computer section, large central book section, front organizer pocket and three attachment points to distribute the weight. If you don't have a ton of stuff, or are on 2 wheels, or won't be carrying the bag for long the Crumpler is nice, but practically inferior to a backpack in the long run I think. Strange but the Backpack is smaller, but holds more than the Crumpler. The XL Timbuk2 worked great on vacation at the beach; held all four beach towels, gallon of water, inflatable toys (for the kid of course...I'm married) sunscreens, magazines, spare clothes and lots of beer. Plus I just filled it with ice to keep the beer cold and it was no worse for wear.
I like the Maxpedition Mongo and the Hazard4 as well but are a bit too tacticool for me (Hazard4 review
HERE)In the end I think a backpack is hard to beat, but it's good to still have a good messenger bag. I'd recommend Dakine and Timbuk2, the Crumpler is not cheap (no good bag is really) but look at used ones on ebay for a good deal, they are built to last and it seems a lot of folks go through the same process in finding a solution for them.