good review....
My opinions , honestly,are these:
I tried boxster (3-4 models, from S My 2000, to the last one S with 310bhp and the 2.9), the SLK, the previous roadster 3.0Si.....I was searching a new second car, so i tried to buy the best my money can buy.so no Sl no 6 series cabrio, no 911 cabrio....and so on...
Using your word:nice like an SLK kinda nice....but only for interior and general refinement.it's only a tad better than BMW.But the new z4 rides better, far better...I don't know if 350 and AMG cars rides a lot better than the 200 kompressor and 280 that I have tested , but they colud not be better by miles....
anyway both cars are not comparable to Boxster....oh dear, if you like also track days and you are a good driver, You will love and buy the porsche.
well refined car,also ...but porsche costs here in italy really more than z4(my z4 costed me 51000 euros, the posche 2.9 with tha same options was almost 13k euros more....)....And to my eyes, the rear of porsche it's ugly.it seems that a giant presed it under his 4 meters feet....I don't like it.one of my first life lesson is:don 't buy something you don't like .
I believe that BMW z4 is ....in between.....
These are my opinions...you will think I am crazy ,drunk(not yet

),horny for my new z4.......Now I want Yours!
sorry for my english....
Francesco from Italy