Official is where it's at.
Although it does take longer to level, leveling isn't like other MMOs where you just mindlessly click, because you actually need a party to make decent experience (for the most part). I don't see any wizards jumping in biolabs planning to solo.
I personally believe party leveling is very fun in low rates because you actually die.
The balance in high rates just does not make sense.
The game wasn't made for people to PVP at level 250, because skill and stat builds get messed up, and certain classes become clearly more powerful than others because of this. Then all it is is who has more MVP cards, and then it's just stupid. Servers can try to balance the classes out but in the end, you should stick to official where the classes are already balanced. Not to mention WoE in official is nothing like WoE in a private server. If you have not played WoE in official (or low rate, for that matter), you have not played WoE at all.
High rates, in my opinion, are so you can play around with having a ridiculous build and trying out different classes and skills, while discovering more about RO you weren't able to see in a low rate because you were too low of a level. But it surely is not worth getting serious about because all in all, private servers are just not dependable. They lag, they get corrupt, they are run by people who have no idea what they are doing, and they close down completely.
Official is the way to go for people looking for a fair, serious game
They just made it free, so now you have no excuse to play a crappy private server.