I may be the target market for the Z4- I wanted a fun weekend and nice day car, looking for more of a touring package than anything hard core. I do like to drive it in a spirited fashion and seek out fun roads to play on all the time.
I originally thought E85 and although the styling wasn't a big pull, it was really the spartan layout and lack of amenities that kept me from pulling the trigger.
Then I thought Mazda Miata MX5 as it was only going to be a weekend summer car. Too small and too plasticky but definitely cheap, even with the hardtop.
Then I drove a Audi TT roadster. Nice fit and finish but the bathtub styling did nothing for me.
Thought Boxster but couldn't rationalize the price hike for a toy.
When the E89 came out, it ticked all the boxes- great looks, hardtop, room for weekend bags, even with the top down. (Golf clubs I put in the E46 Touring), modern amenities like bluetooth, nav, ipod link, gorgeous feeling Nappa leather. I drive the car with top down 98% of the time, but I like the security of the hardtop when parked. It has only seen rain once , in Germany. It's a car two people can ride in comfortably and in normal mode/Drive, my wife makes no complaints about harsh ride. Sport +/paddles is for when I want to feel something a bit more responsive. When the time comes, will have a hard time deciding between Michelin Pilot Sports or the next gen runflats.
Like secretsquirrel said above, for a lot of us, this is the sportiest car we've owned. Nothing wrong with that, and I for one am really happy with it.