Others say...

"Great way to protect your valuable lenses"
This lens case fits my Canon 70-300mm IS USM lens or any one of several of similar size. I attach this to my Tamrac Velocity 7 by the same type of attachment system that Tamrac uses for all of thier products. A flap slides into the available MAS system slot on the camera bag and is secured VERY securely with a double locking velcro strap system. This leaves plenty of room in the bag for my camera and other gear. Nicely padded to protect your lens investment.

"A Solution That Works"
I have been trying to find a way to easily carry a lot of camera equipment for quite some time. I have tried backpacks, shoulder bags, duffel bags, but none of them really worked all that well. Then I saw the Lowepro Street and Field system with completely customizable pouches, vests, belts, and accessories. It works great, you only get the pouches you need so there is no waisted space and you can organize them however you want according to how often they are used (put frequently used items in easier to access places.)

The variety alone of this system is worth the investment, but the quality and ease of use is also very high which is a big plus. You can get a case for any lens, camera body, or accessory you have. I even have cases for my cell phone and water bottle. One word of caution, be sure to measure your lens with the lens hood on backwards if you use them to make sure that the entire thing will fit into the pouch, because sometimes the lens will fit, but then the lens with the lens hood will not. I often carry 5-6 lenses, extension tubes, teleconverters, extra memory cards and batteries, and a bunch of cleaning supplies and other gadgets, which weights a ton. But when they are evenly distributed on this vest/belt Lowepro system it is actually very manageable.

The only down side is that everyone will be staring at you because you have thousands of dollars of camera equipment strapped to your body. But if you can overlook that, this is the way to go. I have used this system at many zoos without ever having problems, in fact just unzipping a single lens and switching seems safer to me than opening an entire bag of equipment every time you want to switch lenses. Many of the cases also come with a built in rain/snow cover so if the weather changes all of your equipment is not destroyed. You can also put just a few pouches on a normal belt when you are just going to take photos in the back yard or don't need to take all of your equipment, which is nice. I really can't say enough about how much easier this makes traveling with a lot of equipment, if you have been hopelessly searching for a better solution, consider this, you won't regret it.