They do drop in value, but so do many other cars. I would not say half of its value in 2 years as a good 2006 Cayman S is still going between 35 and 45 thousand depending on options and mileage. This is not half of its value and it is not 2 years old as the 2010 is already out I believe.
Personally, my stock 987 has been much more fun to drive than my tuned WRX although, I do mill the power a little, but I also have a base model and not an S.
As for the GT-R comments, they are two different beasts. The 987 is an amazing car on smaller tracks, autocrosses, and everyday driving, but it does come short at larger tracks where the 911 is much better equipped to handle it. There is still a decent lack of power and torque, but Porsche is trying to fix that issue. The only thing is that they can only do so much without bringing the 911 down a peg, and they do not want that under any circumstance.