Imagine this. You're at a crosswalk waiting for the light to change. Suddenly, three men (though one of them might be a lady, it's kind of hard to tell with the ski masks) wearing ski masks burst forth from a nearby bank. They make a dash for a black 1986 Chevy Caprice. You have just one chance, one small opportunity to save the day. Pulling your BlackBerry forth from its protective holster, you press the convenience key, instantly opening the camera. A second press focuses and snaps a picture of the car (and its unmasked occupants) just before the vehicle tears away. When you get that photo to the police, you'll be a hero. Those bank-robbing thugs are going to get a little justice - BlackBerry style.
Yeah, that doesn't happen too often. In my experience, it is a rare time indeed that I need to take a picture at a moment's notice using my smartphone. That's why my convenience key was long ago relegated to other functions. Out of the factory, the convenience key on your BlackBerry is set to open the camera. For some that's perfect; for me, that key could be doing so much more.
In my humble opinion, the secret to good Convenience key use is to avoid having it start your most used apps. Think of the five or six (or even ten - it's your BlackBerry) apps you use the most on your smartphone. Chances are, no matter if these are grouped at the top of your All pane or scattered about your BlackBerry,, you can find those most-used apps pretty darn quickly. Why set a key to do what you can already do easily?
Instead of linking your most-used app to the key, think a moment on how you would use the key. You could set the Convenience key to launch an app like QuickLaunch, making many, many more apps available at the push of a button. If you write for the Best BlackBerry blog, you're going to take a lot of screenshots. My Convenience key is set to do just that; capturing what a menu looks like would be impossible otherwise.
You could set the key to start a flashlight application. Or the calculator. Or the... you get the idea. Instead of starting the apps you use most, consider setting the key to open the apps you need most quickly. Perhaps it's not the camera you need at a moment's notice; it's Twitter (RT @Dug SQUIRREL!). The point is, your Convenience key is yours to assign as you wish. Make it something good.