It is my understanding that they do work, but what's the point? You pay more for the fitting, still have to flux and heat the joint. I see no advantage in this. I'd also stay away for the "solder in a bottle" gimmick, too.
I've recently used these pre-soldered Watts fittings from Home Depot. I am certainly not a pro which is probably why I liked them. For me it made the job easier especially when making a connection standing on a ladder in a confined space. Just heat the joint and be done with it. The solder is evenly melted and your hand is out of the way of the torch. The one down side is that you can't just solder to one side of lets say a T-fitting. You must have pipes in all three openings otherwise the solder will melt where there is no pipe and you will be in trouble.
Interestingly enough when I compared prices (strictly at Home Depot)between the standard and the presoldered fittings, there was no clear cut indication that the presoldered fittings were more expensive. Some types were and some were actually cheaper. I don't remember the exact breakdown as to which were cheaper and which were more expensive but I was surprised to see that some were in fact cheaper.
The ones that I saw at HD were about the same price as the standard wrought copper fittings, but they were thinner; less copper, and what I considered very little solder.
They did not leave a nice fillet of solder on the outside that I like to see.