I like to do some research before buying something. There's a certain thrill about gathering information and making the best decision. There's always lots of choices and often the best choice depends on the individual user's needs. After some research, I narrowed down my choices to the Lowerpro brand, and the models Trekker, Slingshot, or Fastpack. Each of these models are available in different sizes. Lowepro is to camera bags is like Manfrotto is to tripods. I ended up buying the Fastpack 200 for these reasons: When I go out on a photo shoot, I will be selective in what gear I need to carry and it will be weight-limited so as not to bog me down. The Mini Trekker carries a lot of lenses, but will probably be left at home to house my gear rather than carried around on trips. The Slingshot is great for fast access but has only one strap which makes it tiring to carry for a long period of time.
Fastpack 200 is just the right size and the right design for me. I can carry it as a backpack. I can have fast access to the side pocket. The general compartment above the camera compartment is useful to carry anything on a day trip, or to carry more gear (like chargers, flash guns, filters, or even a back-up camera). I took it to a wedding yesterday. Left on the floor, I could access the top compartment easily. When following the guests around, I could carry the bag on one shoulder and fast-access the side pocket to grab my camera. Plus, I still have lots of room left over for future acquisitions.