I played on a private server for about a year or so as I was completely against continually paying for a game I already bought (well kinda). It was pretty fun for awhile. The server admin would put together nights where everyone had level 70 characters and tricked out gear and would just PvP all night and you got 1.5 exp all the time since the server population was low and many quests where broken.
That said, while the private server was fun for awhile, it couldn't fix the game itself being boring once you hit the level cap. I personally found it ridiculous to take all the time to put together a large group and do a raid for the tiny chance you'd get a gear drop and it would get dolled out to you and not one of the other 10 people in the raid playing the same class.
The population was also never more than 100-150 people so it was kinda hard to get groups together. It was pretty easy to connect once you found a good server. I thought I remember something about all the big private server people getting sued by Blizzard for copyright stuff but I could be wrong.
Ya only the owner gets sued lol. Sometimes if they are using a Blizzard repack thingy if its diff its ok I used to play but i dont want to pay 15 bucks a month
If you wanna play my serv sometime :P 3.0.3 lol but hey