Interestingly, I bought a z4 3.0si a few years ago, vs my other option which was a new SLK on lease. I now have a Z4M coupe (E86), with no regrets along the way.
My younger brother however, has a big car history at 26 years old. These include the 350z GT (regrets ever moving from), a brand new SLK 250 cdi AMG spec, and now currently a 370Z.
I've jumped between my own cars, and his frequently. I loved the 350Z, it felt really individual, and lots of nice touches inside the cabin that made it feel special. Howeber, as ricey says - getting on a bit and not what you're after.
The SLK I really liked. It did feel a bit heartless, but it was so fast, and imo felt so refined to drive. It was firm enough to make you remember that you're not in a barge, but damped well enough to soak up the bumps! Handling was ok, but you could really feel the weight. The cabin, personally is stunning to be in, and the tech works pretty well too. (don't pay extra for sat nav, buy the add-on unit online for around £200 and the whole thing works as OEM).
On to the 370Z... It looks nice, but as said previously the cabin is pretty noise. This is in part because they hjave almost got rid of the bulkhead, the sub-speakers are no longer behind the seats and the bootspace is practically at one with the cabin. The auto box is nice, and in sport adds some nice blips on the down change. They've made the box slightly aggressive on purpose so that the gear changes dont just smudge into each other. The steering is nice, but the car is way too quiet from the exhausts, especially compared to the 350Z.
So i have rambled, about not very much,. But basically - SLK over a non-M E85/86. SLK vs an M E85/86 would be a tough one for me, purely because of how refined and modern the merc is in comparison. 370Z - you just need to drive one to work that out, as I can't make my mind up and my brother has itchy feet with his already!! Oh, I forgot to say - they really have just raided the parts bin with the 370. The 350 had a lot of really nice bespoke parts, and cost them too much to produce. Jump into the 370Z and my brother was quick to point out that it has the same steering wheel as our mum's Duke, bar the airbag cover! :| :|
e89 - I haven't driven one, but looks alone and the interior
Oh... cayman. Well my friend has a 57 reg Boxter. He's had it for 4-5 years. He's had nothing of the engine failure, but a LOT of niggles from it, ranging from electrics to coolant issues. I've driven it, and of course it is lovely. But, i much much prefer the z4M for cabin quality, design, driving experience, noise etc etc.