I bought one a while ago, before I kegged. When they worked, they weren't bad, but I ran into a few problems. I used the 5 liter minikegs, mostly because you can get them full of Spaten or Bitburger for the same price as an empty keg, more or less.
It is vitally important to make sure that the plastic parts of the bung are aligned properly before inserting into the keg, as they will leak otherwise and ruin your beer.
It is also vital to only prime with the recommended(I believe) 1 tablespoon of corn sugar per keg, else you will overcarbonate and end up with gallons of foam instead of beer when you tap. I learned this lesson trying to split a batch between minikegs and bottles, primed as per bottling, made a mess.
I also used to vent my CO2 manually from the top before tapping, rather than trying to dispense beer to equalize the pressure, lots less foam that way. If you pull the red tab off the bung assembly and then carefully press the inner cap down with say, a chopstick, you can slowly release the pressure. Careful, if you push it all the way in you get a beer geyser.
So, nowhere near as good as kegging, but a nice break from bottling. I really liked bringing them to parties.
I still have my tap and about 10 minikegs, I'd be willing to sell them fairly cheap, if anyone wants them, PM me.