I tested all of the competition (SLK 350, Boxster S, S2000, etc.), and I found the Z4 to be the best in class, especially considering the price. The Z4 has excellent power (and the sound of the I-6 is like tearing silk!), great fuel economy (averaging 26 mpg+ in mixed driving), tight, buttoned-down handling, plenty of room and great safety features.
Powertrain - smooth, sonorous I-6, rifle-bolt six-speed. Heated seats are nice, and I love the long nose, short deck styling.
Ditch the run-flats to improve the ride a bit, switch to hydraulic steering assist a la the Z4M for improved steering feel.