MMORPG Private Servers?

What are your opinions on private servers for various mmorpg's? i play a lot of final fantasy 11 on catseyeXI and world of warcraft on AscensionWoW, and i guess im curious what draws people to private servers or away from them. I very much like ascension wow and catseyeXI for how they change the base games for quality of life reasons in catseyeXI, and ascension wow adds a LOT of new content to the version of wow i enjoy most. For some reason, i just don't enjoy wow after the cataclysm expansion. But also the whole classless concept is super neat too. and they have other servers with other experiences as well.