I really like the side-opening design. I found I didn't really use it as intended--you're supposed to keep the left strap on and swing the bag around to the front of your body--but side access comes in handy under a lot of circumstances. You can easily slide the camera out without having to open the entire top section, and grab media from the camera without removing it from the backpack, and you can even review images without taking the camera from the bag.
I have to admit, though, I'm not really fond of Lowepro's divider system (based on years of experience with an otherwise great backpack, the Micro Trekker.) The dividers are well padded, but I find the flap-based attachments difficult to align properly, the placement of the loop patches (of the hook-and-loop fasteners) too limiting, and the fasteners themselves a bit too tenaciously tight. They stay where you put them, but configuring and reconfiguring the system always feels like a fight. However, many users are fine with the design, so don't take my word for it. In general, it's always a good idea to try to configure any bag before you buy.
When considering the Fastpack 100, think carefully about the size. In some ways, it's perfect; for instance, it holds exactly the amount of camera gear I carry for my routine weekend shoots. However, I also need it to hold letter-size paper, which I crammed into the laptop compartment of the Kata Ergo-Tech Sensitivity V Backpack I used before the Fastpack. It's annoying to have to move up to the larger--and considerably more expensive--Fastpack 250 simply for that flat back compartment. So if you're looking for a daily pack rather than a vacation or shoot-only bag, remember to take into account the detritus of your everyday life. If you don't need to worry about nonphotography extras, the compact and convenient Fastpack 100 should work well.