Well, I just sold my '09 Boxster I had for 3 years and got a 2012 Z435i. I loved my Porsche, and found it to be extremely well built, great fun to drive, loved the engine roar upon smart acceleration, but also the smooth and yet solid driving when just cruising. Great to have two trunks, and the creature comfort items are very well thought out and work flawlessly.
For the money, it is probably THE car to have in terms of the "handles like driving on rails" saying goes... but it is also quite stiff. That means one feels every little bump and crack in the road. And, while some say its design is getting old, it still DOES draw attention.
My Z4 is easily as fast if not faster, handles ALMOST as flat and sharp as the Boxster, but unless one is a demanding track enthusiast, it works extremely well, looks great, and can get you into deep trouble very quickly if you don't apply wisdom behind the wheel! It's also a very comfortable cruiser. And yes - comparably equipped, it is less expensive than the Porsche. And BMW often promotes pretty decent deals -- I got a nice windfall from selling my Porsche and am now enjoying a very affordable lease with my Z4.
I'm glad I had a 3-year Porsche "experiment", but I'm very happy to be driving a Z4 35i now, too!
Doug