I purchased the Lowepro Fastpack 250 about a year ago to use on photo walks. The larger bags fit more gear, but I wanted something that could hold a decent amount of gear without too much work. I really like the bag for how convenient it is, and it is a high-quality product.
How much does it hold?
The camera compartment will hold a large DSLR with a battery grip attached without any problems. Also, I like that this bag allows photographers to keep a lens attached to the camera and still fit it in the bag. The slot for the attached lens is quite large, but not quite large enough to allow me to keep my 70-200mm lens attached to the camera, even if I reverse the lens hood. That's really unfortunate, but it would easily fit lenses like a Nikon 70-300 or a Canon 75-300mm.
Aside from the camera/attached lens compartments, there is ample space for several smaller lenses in the side compartments. Wide-angle lenses or 50mm lenses could be stuffed into several spots.
What I really like about this bag is that it has an open pocket at the top of the bag where you can throw in a whole bunch of stuff like flashes, a small flash diffuser, batteries, remote releases, etc. You could even pack a little lunch in there or keep your rain protector in there.
There is also a spot for your cell phone on the straps of the bag and a water bottle holder on the side. Ample space for filters can go in the small back pocket, and memory cards have a special pocket just for them. Oh, and there is a handy slot for a laptop, too!
What makes this bag unique?
The most unique feature of this bag is that it has a convenient side-access pocket so you can get your camera and attached DSLR out of the bag without taking the bag off your shoulder. This is a big win and probably the most attractive feature of this particular photobag.
Who is this bag for?
I find it best suited to hobbyist photographers who don't have large lenses and gear. For me, this is a perfect bag to carry on a photowalk because I can hold a lot of gear without weighing myself down; however, it really doesn't work for me as a day-to-day photo bag because it doesn't fit my professional gear like the popular 70-200mm lens.
How much does it cost?
The bag costs around $75 at many online retailers. It's probably more if you buy from a brick-and-mortar store in your town. Here's a link to buy it from Amazon.com.