I followed this vehicle since the X Coupe Concept unveiling in Jan '01 and loved the look the moment I saw it. Previously owned a '00 Z3 M Coupe (S52 engine) and would rate the Z4 similarly but with welcome upgrades in chassis stiffness and cabin noise reduction. I-6 is inherently smooth, mechanically sophisticated, and refined but not sleep-inducing. Much better than the M3's S54 which I found to be rough, unappealingly raspy, and difficult to drive smoothly in traffic. Materials quality and fit and finish is fine but doesn't approach VW/Audi. Great steering feel and response. Somewhat heavy but appropriate clutch and shifting.
Engine performance and refinement is stunning (second only to Porsche) and it's the main reason you buy a BMW. Steering feel and response provides superb cornering and handling. Stereo sound is fairly good, but probably not best in class.
Ingress/egress is somewhat awkward but perhaps part of owning a small, low- slung sports coupe. M-Sport seats are a little narrow and thinly padded even for a skinny guy. Make in-dash CD a six-disc unit; add SD card capability. Door map pockets are so inadequately shallow, why bother?