RYU, congrats on the new ride! It's looking beautiful:biggrin: I never did get my callback on my black limited that I had on preorder, though 
As for the cayman, I got a manual and have been DDing it for some time now. I actually jumped into the NSX for the first time in about 2 months last week. Suffice to say, I was kicking myself about 2 minutes into the drive for not getting out in the NSX more with the great weather we've had lately. Anyway, onto the differences (in my personal opinion), at least in day-to-day driving:
1) Controls/Ergonomics/Etc.
We all know about the unique driving position of the NSX. The Cayman is much more 'traditional' with a lower seat and higher beltline. I feel both cars have pretty good visibility, with the NSX having better frontal visibility but the Cayman having better rear visibility. The hatchback and attached rear deck, combined with the lower seat and extra headroom makes the Cayman feel MUCH larger inside than the NSX. It's still very cozy, but the "airiness" of the Cayman cabin is nice at times, especially with passengers.
2) Clutch/Shifter
The pedal feel on the cayman is much heavier. At first this bothered me, but having gotten used to it and actually prefer it to the NSX's pedal. I think if you're sitting in traffic, the NSX pedal may be better though.
The shifter in the NSX is very short, with a narrow gate, and what I'd describe as a bolt-action. I also have a SSK and counter-sunk knob for my NSX, which exaggerate these characteristics. My Cayman is a 6 speed, but doesn't have the optional SSK. As a result, the throws are longer, and the gate is wider as well. This makes every shift feel more deliberate, but the shifter is damped in a way that make you enjoy shifting just the same. It's not quite as good as my old s2000, but it's in the same league.
3) Steering feel
My NSX has all the noncompliance goodies, chassis bars, swaybars, and coilovers, and is generally set up for a somewhat aggressive ride. I feel everything (and not in a bad way), turn in is great and feels sharp/quick, etc. My cayman has a refreshed stock suspension (from PO). Compared to the NSX, the steering is heavier weighted and doesn't feel as though it's boosted (I have an NSX with power steering). Turn in doesn't feel as sharp/quick with the Cayman, but I feel like it may actually rotate more easily at will. The rear suspension geometry doesn't feel as supple, especially when you feel hops mid-turn due to poor road surfaces, but I learned to trust it and it's VERY stable.
The NSX seems to make you work for things and is very rewarding along the way, where the Cayman seems to do them just as well, with possibly less effort, but less drama too.
4) Engine
My NSX has a unifilter and AP-X exhaust, but is otherwise stock. The Cayman S has a softronic ECU flash which enables 'sport' mode, essentially quicker throttle response that would come as part of the PSE package.
When I first got the Cayman I loved the powerband, it feels like there's more torque, and it's very easy to keep the powerband low. Like the V6 in the NSX, the flat 6 in the Porsches is something special when it comes to the soundtrack. The Porsche seems to operate in different octaves depending on RPM, all of which are pleasant. When the NSX's V6 is singing, it sounds amazing, but the drone under 3k is not as pleasant, at least to me.
After driving my Cayman so much, I'd really gotten used to the driving characteristics of the car. Perhaps the biggest surprise for me was getting back into the NSX and realizing, "WOW THIS MOTOR IS SMOOTH, AND THE POWERBAND-- IT'S SO, SO, LINEAR!" This may sound as though it conflicts with my sentiments about the Cayman, and its great, usable powerband, but it's really just a testament to what a gem the NSX's motor is.
Despite this, at least for me, I drive these cars very differently. I shift in my cayman at just under 3k, while I'd shift in my NSX at around 4.5k (for non-aggressive driving). I also cruise on roads at 45 in 5th (or even 6th) gear -- which can be as low as ~1500 rpm -- in the cayman, but wouldn't consider doing that in the NSX. As linear as the NSX's V6 is, it still feels a bit gutless at < 3k. Combined with the drone, keeping the NSX at higher RPMs is easier, more convenient, etc.
5) Overall experience
The Cayman is a great car, but the NSX just feels more special. There are tons of people who drive their NSXs daily, and I could be one of them, but honestly don't think it's best suited for that. The Cayman has easier ingress/egress, MUCH more cargo room (and interior storage!), a larger cabin, has no issues with driveways/speedbumps, and just generally feels like a more 'normal' car. Its popularity and dimensions also make it less of a 'risk' with all the soccer moms out there.
For me, I feel I have the perfect combination of a fun daily driver that I wouldn't cry about if/when something happens to it, and an amazing, very special car to take out on weekends. While their performance is similar enough for me to call it a wash, the driving experience and characteristics of each car shines through with their intended uses.
