I've used them before. Yes, you have to prime them and still use co2. Not using CO2 will cause the beer to oxidize and thus ruin it.
They can be handy, but over time, I grew tired of them. I had several that nearly explode and one actually leaked. I did not over carbonate. The kegs just can't take a lot of pressure. The bungs were always a hassle to place and remove. Cleaning is an issue as well because the opening is very small. I now have a kegging system and it was something I should have done a long time ago.
If you can't keg and still want to use the mini kegs, some suggestions. Don't use a mini keg more than 3 times as they begin to rust. Use PBW to clean them and rinse well. Use a hair dryer to dry the inside. Still, it is nearly impossible to completely dry them, thus the rust. Use keg lube when inserting the bungs. Use carbonation drops for priming. It's easier to be exact on priming. And finally......Do whatever you can to keg the traditional way. It's worth it, believe me.
Cheers!