These fittings seem to be a lifesaver. Sweating pipes that have moisture in them is one difficult task. You can jam a piece of bread in the pipe to block the water, but it doesn't always work. The Sharkbite fasteners seem to help where conventional or PEX fittings fail.
In my expeirence a few years back as a plumber's assistant, these are GREAT for just making a couple of connections but you wouldn't want to plumb your entire house with them due to the price. Particularly if you're connecting copper to pex or PVC, but also handy if you are re-routing copper pipes or connecting to existing copper. As most people know, as you solder the pipe, you must always leave one end open so that the expanding air can escape from inside the pipe. This presents a problem sometime when re-connecting new plumbing to the old piping unless you have a valve you can leave open or something that allows you to leave that escape route for the air as you solder.
Even though the existing plumbing may be turned off, dry inside, and very well cleaned, if there is no place for air to escape, your solder will "blow out" as the air heats up inside the pipe and after the water is restored, you will end up with a leak (which is now MUCH harder to fix because your pipe is full of water again.
Anyway, these fittings are awesome for such an application, plumb up your new system, sweat all your joints, then push the sharkbite fitting onto your old plumbing and then your new plumbing and presto!
Focus on Fundamentals consists of 45-minute on-line classes that cover a host of technologies. You learn without leaving the comfort of your desk. All classes are taught by subject-matter experts and all are archived. So if you can't attend live, attend at your convenience.