I used to never agree with paying that kind of money for defenders. Always felt the large fees should be reserved for attackers.
However, this was probably due to being brought up on British football where there was never any emphasis on defenders being skillful. Players like Puyol, Yaya Toure, Coloccini and the more continental style of CB/DM show that the key to attacking is often the defences distribution. With that in mind Luiz would be great at Barca, he's not good enough to command that kind of fee though.