I'm using a Surge now, but have used a STM Revolution (Medium) and a Power Pack by Outdoor Products (
linky) in the past several years.
I started with the Power Pack, which had TONS of room for pretty much everything under the sun. The main compartment was spacious and was very convenient in that it included dividers for keeping books and notebooks separate. Lots of spaces and designated pockets for thumb drives, pens, pencils, calculators, a mouse, etc. This
site has a better view of the different compartments. It had two generously size side pockets for umbrellas, water bottles, etc. And the laptop compartment was very nicely padded.
The STM revolution was very nice, and the included rain cover saved my laptop at least a couple of times. That said, the main compartment, specifically for carrying anything more than a single book and maybe a couple notebooks was pretty tight. It's very light and feels very mobile... also has a decent amount of room for pens, pencils and calculator but at the expense of overall carrying capacity. The cushioning in the straps is noticeably thinner and less forgiving than on the Power Pack.
What led me to the Surge however was that the Power Pack would fill up and get very heavy and bulky very quickly. When I downsized from a 13" MBP to a 13" Air, I went with the STM Revolution. However, the STM began to get quite limiting space-wise, so I moved up slightly in size to the Surge. To me, the Surge is a nice compromise between the previous two. Padding in the straps is in between the STM and the Power Pack, with interior space approaching that of the Power Pack.
TL;DR
Power Pack
+ SPACE! Interior organization, very comfortable padding in straps, has designated pockets and slots for all of your computing and class needs.
- Can get bulky, and heavy.
Recommended use: If you're going to be out of your dorm/apartment all day and need a bag to carry everything you need.
Revolution
+ Mobility, rain cover is a lifesaver.
- Space, padding in straps less forgiving for all day lugging around.
Recommended use: If you go the e-book route for text books, would be ideal size/form for biking to and from classes.
Surge
+ Space, without getting bulky. Mobility, without sacrificing too much space.
- At least where I am (University of Michigan), it seems like 50% of the campus has the exact same bag. If you care about things like that.
Recommended use: Compromise between space of the Power Pack, and the mobility of the STM.