In 05 when I was shopping for a new sports car, I test drove the Z4 along with a Boxter, Honda S2000, Nissan 350Z, and a Mazdaspeed MX-5.
I ended up with the Mazda. (Still have her). I found it to be the best handling of the bunch and I preferred the styling, especially the interior.
I spent a couple hundred bucks on some minor bolt on performance tweaks and she makes about 220 HP, so she keeps up with the Boxters and Z4's just fine.
Doesn't hurt that Miatas are basically maintenance free. Change the oil and most will go 200K without any major expenses.