I buy cans of compressed air by the six-pack. It's good stuff, but not for everything.
One of my favorite uses for canned air is to help with cleaning camera bags. When you work outdoors, dirt and dust can inevitably find their way into your pack. Sometimes, it's just the stray particulates blown by the wind or carried along for a ride when you return your gear to your bag. Sometimes, it can be more obvious, like when I flipped the cover back over my pack, and it acted like a scoop to lift a lot of dirt all at once. That was fun. But no matter how careful you may be, at least some foreign particulate can end up on the inside of your bag rather than outside.
I always make a point of cleaning my gear when I get home from a trip. For camera bags, this starts by removing everything and shaking the bag upside down. All sorts of stray things have fallen out over the years. But the nooks and crannies in the corners can still hold surprises, and a few quick shots of compressed air can make short work of getting the final bits. It's worth the effort to be thorough to avoid random bits from working their way into your camera and lenses. It's one thing to get dirt in your bag. It's a far worse problem if it gets inside your camera.
With care, compressed air can also work well for getting stubborn dust off the outside of your camera and other gear. Any texture or crevices can provide a safe harbor for dirt and dust, and a quick blast with canned air can send it flying.
But you have to be careful when using canned air. The reason why it works so well to blast foreign matter away is that it is highly pressurized. And most of that pressure comes from a chemical propellant used in manufacture. Spray lightly with a can of compressed air, and you get a powerful burst of air. Shake the can and spray more heavily, and you'll also get a spatter of gunk. Keep that in mind whenever using this stuff. Spray carefully, and never spay anything sensitive or fragile.
Sometimes, you hear of people using canned, compressed air to blow dust out of their camera. Perhaps some of these cautionary tales are mere folklore, but at least some no doubt retell actual events. The stuff works so well for getting cookie crumbs out from under the keycaps of a laptop keyboard, so why not a quick puff to clear out dust before it finds its way to your camera sensor? Well, the high psi air burst can break the mirror box, shutter, and associated components of your camera. And should you be lucky enough not to damage anything from the pressure, you're sure to end up with propellant spatter on your sensor eventually.
The truth is, those cans of compressed air contain mostly propellant. That's why the cans come with an inhalant abuse policy statement. The propellant, typically difluoroethane or similar, is known to cause cardiac arrhythmia that can be fatal. There is no "air" in there at all. As you press the spray button, the propellant shoots out and instantly starts to vaporize. As it does so, it pushes the surrounding air along for the ride. If you're lucky, the propellant is all gone before the burst hits your target, leaving you with nothing but air. You're taking a bit of a gamble when using this stuff, but it works wonderfully for the right uses where your risk is low. When misused, your chances become more dicey.
By the way, difluoroethane is also used as an environmentally friendly refrigerant. A great deal of heat is absorbed during the quick vaporization process. That's why freezing your fingers is a sure sign that you're going overboard with the compressed air.
If you really must blow something out from inside your camera or lens, invest in a Giotto Rocket blower or equivalent. With its large rubber bulb and a air tube, a simple squeeze on a Rocket can generate a good burst of air that avoids the excessive psi and propellant found in commercial canned air. Be sure to hold the camera body upside down, with the opening facing the floor, to increase the likelihood that anything dislodged falls free and clear rather than just landing somewhere else inside. As a cheaper but slightly less effective alternative, buy an earwax removal syringe from the local pharmacy.
And never attempt to clean your camera sensor unless you know what you're doing. Damage it, and you're in for an expensive repair bill.