They don't contain any CO2, which is a common misconception. They are simply a hardened, premeasured slug of sugar and carbonate the beer same as powdered or liquid sugar would.
Well, I've only used them a couple of times, but I don't think I could notice a difference in time to carbonation... They're very handy when you only want to bottle a few and keg the rest.
About the same time to carbonate, since it's the yeast that do the work. They are also handy for people, like me, who don't bottle much and for saving a flat batch.
I have just bottled my first brew and realised that I have only added one carb drop to my 740 ml bottles when the recommended dose is two drops. What should I do to fix this?