I had one for 11 months, and used it for long distance driving, a role it does very well but so would a 330D. The engine is not gutless at all but very linear and ultimately blunted by either 6 speed manual and the kerbweight. Mine was a DCT which IMO is a better combination, due to the better spaced gearing and the truly excellent shift speed and extra ability it gives (no comparison with the SMG3 gear I had in my E60 M5).
The engine is muted, but the "official" exhaust mod fixes that for £200 (Google "Paul the Welder" and 2 pipe mod), giving the exhaust sound it should have had from the factory.
What put me off mine was the use I was subjecting it to, with its prodigious thirst meaning a 400 km autonomy becomes a bit boring, as I was usually in for a 500 km stint. Also, the interior is quite plain, recommend searching for one with extended leather and upgraded audio and with the new CIC nav system (or one without, as the CCC one is quite outdated, slow and not intuitive to use).
The Z4M is, however, IMO a more special car, raw and analogue, and its compact size and 2 seater setup makes it a more interesting car for an occasional weekend car.
Finally, I was also scared off by (mostly US) stories of rodbearing failures, same as the E60 M5, as the engine is essentially a 4 litre V8 version of the 5 litre V10, with 2 cylinders chopped off, and that engine sees quite some failures. An AUC warranty would take care of that problem, of course.