Greg - I am subscribing to this thread to read other comments. I have been torn between a Cayman, Cayman S and Elise. I have driven an Elise, Cayman S and C6 Z06. In each case it was a test drive with the longest being in the Elise. I will add my .02 worth for now, but understand this is based upon limited seat time.
Elise - The best 4 wheel sport bike on the planet. I have only driven the base Elise not super charged. The car has cat like reflexes, better than any car I have ever driven. The turn in, braking and acceleration are immediate. While not as fast as a Cayman S or Z06 it is still my favorite. I prefer the nimble handling over power and I think it would be very reliable. On the down side the ingress / egress is a pain but once in the seat you know you are home. Luggage space is almost nonexistent so if you wanted to do a weekend getaway you will need to pack lightly. I can only imagine that given the current economic situation this car will go down in value, good for us who want to buy one. Here is a picture of my favorite color; Polar Blue.
Cayman S - The Cayman seems to reward you the faster you go. It seems like the harder you push it, the faster it wants to go. This can be a double edged sword on the street. With the Elise you can feel the speed where the Cayman insulates you from it somewhat. The S has much more grunt, a more flexible / usable power band on the road. The Elise requires you to work the gearbox to be quick where the Cayman seems happy in almost any gear. If my plan was to drive cross country then the Cayman has a much better balance of comfort and speed. You can tell the weight difference between the Cayman S and Elise when driving. The Cayman has a much quieter cabin and a superior sounding engine. When the Cayman is running full song, there is no mistaking its' heritage.
C6 Z06 - Unbelievably fast is a straight line, challenging to toss around due to weight and over powering the tires. It does not seem as comfortable at speed as the Elise and Cayman. It tends to feel floaty when you hit the higher speeds. Not the best interior and the fit and finish is not on par with the Cayman. I would not turn one down if given to me but I see it as more a 1/4 mile or top speed car. While it is an excellent handling car I feel it is not equal to the Cayman or Elise.
All three are fantastic automobiles and 15 years ago I would never have dreamed of the options we have today. All are faster, safer and cleaner than anything from the past.
Like I said in the beginning I am anxious to hear what others have to say.