Personally, I think the Z4 is severely underrated in terms of cars in this segment. Everyone picks the Boxster by default (justified) but the only thing worse than a BMW option list, is a Porsche option list. I wouldn't consider the TT or SLK.
Honestly, I'd pick the Z4 over the Boxster. I know that's heresy, and blah blah driving dynamics blah bah
I got real interested in replacing the S2K with a Z4 - I was trying to decide if having something the wife could drive would make our car logistics better (I'd get it with DCT), but ultimately that's not what limits us the most, it's the lack of back seats (so going to back to my love of muscle cars + back seats == Mustang)
It's also hard to beat the unfortunate reality that it's not fun to HAVE to wind it out to 9000 RPM to pull away from every stoplight, every time.
It's great an "extra" car, one you take out to wring out