When my husband and I were looking for another car three years ago, I did some cost comparisons on the dealership (Saturn at that time) as my Aura was being worked on. Went throught the lot noting year/model/miles/amenities noted for about 12 used cars.
Exterior styling
The 2006 Z4 is the classic styling that has always gotten a person's attention. Squared rear, rounded front with the classic 2-part grill. The convertible top is a light brown in color which makes it standout from the other black top models seen.
Interior styling
Though the Z4 has a lot of features, the dashboard is simplistic to look at, easy to use.
Front seat support & comfort
I find the front seat, even heated, to have great lumbar support, my sciatica does not bother me even when I have to use the clutch. Those with long legs are amazed at all the leg room available.
Fuel economy
Around town I get about 23 mpg but on the open highway, get 30 to 32 mpg which is huge since the Z only uses premium grade gas.
Ride smoothness
due to the tires (high performance) width, the ride can be a bit jerky if the road are warped but on a smooth highway the drive/ride is unbelievable. The Z typically comes with 'run-stop' tires which were really a very hard ride so that is why I changed to traditional tires. The only problem will be if I ever get a flat, there is NO spare tire.
Went home and did some Consumer Reports comparison of the cars and the 2006 Z4 3.0i was a best buy for that year. the price Consumer Reports suggested was more than the dealer wanted and the mileage was low (12,800) so we decided on this one. Love 6 speed but husband had to 'relearn' manual.