I previously owned a 2003 Z4 3.0, and now have a 2006 Z4 3.0si. While I loved the first one, the new Z4 is spectacular. Press the sport button on the si, and it changes the manners completely. I always felt that I could have used a little more power- but not so much tha I have no choice of transmissions and pay a gas guzzling tax, like the M. This is that perfect car. Until the twin turbo from the new 335 makes it over!
I love the paddle shifting on the wheel, the sound system really rocks, and the M seats (a totally worth-it option).
Where is the lock (so valets cannot access it) for the glovebox- or better, the center rear storage compartment? Give me a lock there, please. Also, the motion-sensing alarm cost $1200-$1400, so make sure you negotiate that at the beginning if you want to park with the top down and have security.