Well, people do use them, so they can't be that bad.
The biggest drawback seems to be that you have to be more careful with them when you prime, as it is possible to damage them with too much pressure and you can't use the harsh chemicals some of us are fond of.
If you don't mind carefully measuring your priming sugar and you are willing to use iodine or oxygen based preparations to sanitise, then I don't see too much of a problem. Some people do swear by them.
I'm not about to sell of my cornies and start using minikegs, but for some people they are probably a good fit.