I have been using a Lowepro Micro Trekker 200 for the last several years. It's been a great bag and has held up very well. The problem is, it's just not comfortable enough to wear for an extended period of time. I usually set it down somewhere (leave it in the truck). The camera comes out and gets hung around my neck. If I need to switch lenses, I'm out of luck unless I walk back to where ever I set the bag down. I wanted something easy and comfortable that would allow me to carry just enough equipment to get the shots I wanted.
After doing a lot or research on line and in store, I found the Slingshot line of bags. I chose the 202 because it was the smallest bag that would fit my camera with a mid range lens attached. My go-to setup is a Nikon D5000 with the 16-85mm lens. As small as this combination is, it won't fit in the 100 series bag unless you detach the lens first. I chose the 202 over the 200 because the 202 has a strap to carry a tripod built on to the side of the back pack.
**Update 11/12/10 - I recently purchased a Nikon 70-300 mm VR lens. The lower camera portion of the 202 will fit the equivilent of 1 DSLR and up to 4 Nikon 70-300mm VR lenses and still have enough room for a charger & spare battery. (I know there is no reason to carry 4 of the same lenses. I share this because it gives you a maximum point of reference.) The 70-300 lens attached to the camera will fit comfortably. The other 3 will fit, but tightly! The equipment I use most often is a D5000 DSLR, 12-24mm, 16-85mm, 70-300mm, 35mm F1.8 and an SB800 flash. You can comfortably carry one DSLR, any 4 of the above, and a battery charger & spare battery in the lower portion of the bag. There are still front and top zipper pockets for other gear and/or your lunch. - End of update.**
I also bought the Lowepro Bottle Bag which I highly recommend. It attaches via built-in straps on the side or back of the slingshot. So now, I can even carry a drink. Lowepro sells a variety of accessories like the bottle bag that can attach to their camera bags.
From a pratical standpoint, the Slingshot series of bags allows a photographer a considerable amount of convenience and freedom. The bag is comfortable on your back. You can carry as much or as little equipment as you want. When you need it, the bags "slings" around to your front without taking it off your shoulder. You pull out your camera, take your shot, put the camera back, and move on hands free. With the bag in the front position, it even makes a steady spot to rest your arm/support the camera for those longer exposure shots. The Sling Shot even has a built-in all weather cover that you pull out to protect the back pack and your equipment in the event of rain or snow.
The one odd thing I found only affects you if you buy the bottle bag or some other accessory like I did. The only logical place to attach the bottle bag and be able to easily reach it, is on the same side where the tripod strap is. (The other side of the bag has the flap that opens to the camera.) If you do this, you can't carry both at the same time. You can attach the bottle bag to the front (back of the back pack when it's on your back), but you'll never reach your water bottle without taking the back pack off or "slinging" it around. This is not a problem or a design flaw. It is just something to be aware of if you take my advice and buy a bottle bag or other accessory.
The one complaint I have... The opening where the camera body sits is considerably wider than it needs to be. It turns into waisted space. Lowepro should come up with a use/design to take advantage of this wasted space. If they are reading my review, it would make a great spot to install a small compartment/wallet to hold spare batteries and a charger, add one more divider to hold something like lens hoods, or extend the existing divider so you could fit a longer zoom lens.
In the reseach I did, I couldn't find a better back pack. It is so near perfect that unless something better comes along, I have to give this bag a "highly recommended" to anyone considering it. I would happily buy it again.