I test drove an MX-5 (PRHT) today and it was great. MX-5 is a little more snug fitting than the Z-4 (in the cockpit as well as about half the trunk space) and the ride is smoother (less choppy) in the Z-4. I live in PA and the roads suck. This will be a summer fun car - stored in the winter.
Of course, the MX-5 is about 15 grand less, I can get a true red (Z-4 has an "orange" red - pretty crappy). I'm guessing the cost of maintenance on the Mazda will be much less over the long haul. The BMW dealer has a horrible reputation but I did like the way the Z-4 felt. When driving both cars "hard" it was a toss up...normal driving and the edge goes to the Z-4 - it's just more comfortable. I think with the top up there was better visibility in the MX-5. And you gotta love the PRHT although seeing the red top (instead of black) does take a bit of getting used to.
I want to love the Miata but there was something about the Z-4.
Convince me to go with the MX-5.