At first private servers sound great! Free, you can play 90, do all you want.
exp
Then you realize that there are many bugs. Also, GM's tend to run them like banana republic dictators. Last, they are dependent on server costs and the more people play, the more expensive. Also, good servers are very expensive. I've lvled all the way to 90 on a private server, trust me you don't want to play private server it's not a good experience long-term.