I traded in my 05 525i on my new 06 530i. The increase in horsepower to 255 makes all the difference in the world. I've got over 2000 miles on the new BMW and the fuel economy is even better than with the smaller engine on my 05. I'm averaging 23 mpg city/hway combined and over 32 mpg on the highway in cruise control. The performance of the new 06 is far superior to the 05 and the fuel economy even better.
Premium sound system is awesome! The ride and handling of the vehicle is even superior to the 05. I-drive is fine, once you've learned the system. Blue Tooth works perfectly, as well.