After reading numerous positive reports about the Fatboy, I finally found an excuse to buy one. I'm headed to Costa Rica in late March and I figured it'd be great for short hikes or walks around the city. I haven't had an opportunity to actually use it yet, but just filling it with stuff and walking around my apartment has made me really love it. I also wanted to add to what Ritt had said. He stated that the new S-type fatboy has a loop on the back for attachment to a belt. I just wanted to add that the "old style" fatboy has the same thing. I never understood why some women love purses and bags, but now I think I understand.
I have two other bags that are similar. One is a fanny pack and this I use for the gym. I live in manhattan and I don't like to leave stuff locked in my locker at the gym, therefore the fanny pack carries my iPod, workout gloves, apartment keys, gym ID, and a couple of dollars. I wear the fanny pack under my t-shirt and don't worry to much about looking like a dork in the gym.
My other bag is a camelbak Hawg. I got this as a day pack for hiking (for around here and also for Costa Rica). I figured this would be a step up from the Fatboy because it has much more carrying capacity. It could carry a couple of lunches (for me and my girlfriend), a raincoat, longsleeve shirt, first aid kit, etc. Basically, it could fit much larger things or just a larger quantity of stuff.
Overall, I like the Fatboy and Hawg a lot, but will have to wait to provide a further more in-depth review (as I haven't gotten to extensively use either yet).
I guess I still see the usefulness of a fannypack, but I like my fatboy and camelbak much better.