Here are the PROs and also what I was able to fit into this backpack:
-Largest Compartment: has a padded laptop section with a wide velcro strap. In this laptop section, I place my wet/dry bag containing 2 cloth diapers, changing pad, and baby wipes container. Inside this compartment but not in the laptop section, I put a clutch purse, nursing cover.
-2nd Largest Compartment: baby blanket, change of baby clothes, rattles/toys, snacks.
-Top Small Compartment: keys, cell phone, camera
-Bottom Small Compartment: pens, napkins, plastic ziploc bags
-Side Mesh Pocket: burp cloth, bib, cloth reusable bag.

This backpack only has 1 external water bottle mesh pocket. It would be nicer if there were 2 external velcro or mesh pockets.

Since most of the functional and stylish diaper backpacks are around $100, my husband and I are so happy that this backpack fits our needs.

As a regular backpack for a student:
-thick, padded laptop section. but, the bottom of the backpack is not padded, so would suggest placing a blanket(?)/some-kind-of-cushion for your laptop.
-large compartment: if you have a thick, old-school-like laptop, then you probably only have space left for a filled regular-sized 3-ring binder.
-2nd largest compartment: lots of space for about 2-3 regular textbooks
-top small compartment: keys, camera, mp3 player. although small, there's alot of space for personal items
-bottom small compartment: has a pocket, but this compartment is shallow. you can't fit more than maybe a PDA/day planner, some napkins and some pens.

5/23/2013 update:
Now with 2 kids, it gets a little tricky with one backpack and 2 little ones. Child #1 is 2yrs old and child #2 is 4months old. Also, we currently use cloth diapers at home and disposable diapers when not at home. Our cloth diapers significantly added bulk (but not weight) to the backpack.

-largest compartment: all for child #1. the laptop section contains disposable diapers, wet/dry bag, wipes case. the non-laptop section contains spare clothes, sippy cups, 2 large thermos (hot water, cold milk, don't ask), small reusable bag of toddler utensils, Fisher-Price high-chair seat cover.
-2nd largest compartment: all for child #2. spare clothes, bibs, SkipHop changing pad, disposable diapers, toys. (we place the formula container and bottles in a long strap collapsible insulated lunch bag)
-top small compartment: camera, raisins, toddler socks. there is space for a couple more items.
bottom small compartment: reusable bags, pens. there is space for a couple more items.
-mesh pocket: baby bottle, bib.

Although I can fit more things until the zippers are stretched, I just choose not to. I know I can probably fit all the baby bottles and formula container somewhere in the backpack, but then accessing the other items would require unloading some of the backpack to access the areas at the bottom.

By the way, I bought this backpack at about $27 during a slow time (not during back-to-school times), so this backpack has become a huge lifesaver at a super low price.

3/4/2014 update:
Still using the same backpack! I'm a Jansport fan FOREVER! Never in my wildest dreams did I ever anticipate that I would be using a Jansport backpack as a diaper backpack. Anyways....
-Large section contains: 2 high chair seat covers, 2 IKEA toddler bibs, reusable sandwich bag of scissors (cut food) and toddler/baby utensils, "emergency" snacks, Thermos of hot water. Laptop section carries disposable diapers and wipes container.
-2nd largest section contains: Thomas train travel set, mini Look and Find board book, crayons and coloring book
-Top small section contains: socks, misc.
-Bottom small section contains: shampoo (for messy restaurant disasters), several reusable shopping bags
-Side mesh pocket: bib/burp cloth (for runny noses), napkins.
*I carry separate insulator bags for the formula and milk for 2 kids. Yes, it is alot of gear but I have not yet figured out a simpler solution. Clearly my own incompetence. Remember to use your car as a second diaper backpack with emergency toys (BALLS), spare clothes, diapers, socks, thick hooded jackets. Believe me, I REALLY wanted to be that spoiled stay-at-home mom with the Louis Vuitton or Gucci diaper bag, but their bags just AIN'T good enough. Its JANSPORT ALL THE WAY. I save that money for Costco errands ;)

6/16/2015: I'm still using this same exact backpack still in the same exact way as described above. :)

4/12/2018: Kids are now 6 and 5 yrs old. We don't use this backpack much anymore for our daily needs. Since both are potty trained, there's significantly less to bring. Also, we don't want the kids to get comfortable that we carry everything for possible pee accidents. Anyways, we still use this backpack in the summer for spare clothes after swimming. Also, this backpack is still used when we are on vacation / traveling. We have 2 of these backpacks and only sent in one for repair of a damaged zipper. Otherwise, still a phenomenal backpack for carrying drinks, snacks, spare clothes and shoes,  tourist guides, and reusable bags. If you ask other parents at this point, it is likely that none of them are still using a baby bag. This backpack is still the best baby bag and these transition into a regular family backpack due to durability, versatility and lightweight nature. Yes, those Petunia Pickle Bottom backpacks are ever so fashionable and beautiful. However, long term multi-purpose backpack will make your wallet smile every time.