Daemocles

You know, I have been playing WoW for over 2 years, and have never even tried a private server. Aside from the cost factors, what reasons are there for using one? I admit I am nervous about it. Do they keep their servers up to date? Does it affect an account? I know why Blizzard upkeeps servers, (they make money), what motives do these private servers have for what they do? Which ones would you recomend, and why?