Update on Cab 4S X51:
This is still very early, I've only got about 600 miles on my car, but thought it would be worth sharing my experience so far.
The first and most surprising thing is how comfortable this car is, as a matter of fact it has gotten even more comfortable after a few hundred miles. I suspect that some of the suspension was still somewhat stiff when new and by now it is settling in. It absorbs ondulations on the road and bumps pretty well - it just doesn't like pot holes.
If you have the Cab you have to use the windstop. I went out with my wife a couple of weeks ago and she was complaining that she had just taken a shower and that her hair was wet and why did I insist in having the top down. I told her it would be comfortable, but you know how it is, at that point you have no credibility. Ten minutes later she surprised me telling me how comfortable it was and that she couldn't believe we were driving in a convertible. It really works...
The performance is incredible. I'm trying to limit my revs to 5000rpm, so I know I'm not getting the full flavor of the car yet, but the gears are short and if you step on the gas you're changing the gears in pretty rapid succession up to 5th gear (by then it is not trying to limit the revs that hold me back - it is the fear of getting arrested! - LOL!)
The torque and the traction coming out of the curves is an experience that no other car provides. My wife has an SL500, which is a pretty nice car, but after driving the Porsche, it felt like I was driving a truck. I don't want to bash the SL, it is just that the Porsche provides such a great driver involvement that very few cars can compare.
The handling is superb - this last weekend I took some nice twisting roads and it felt like I was dancing with the car going from one curve to another at pretty good clip.
I was very lucky with my transmission. It is soft as butter and I can change the gears with the tip of my fingers and they just seem to be there. I only missed a couple of gear changes, both times going into 6th - and it was my fault.
Well, how does it compare with the regular engine? I spent a couple of days at the Porsche Driving Experience in Alabama, and the car (a C2S) felt great, so I'm sure that I would be just as happy with a regular engine, but I don't know if it is the engine or the PSE, but downshifting in this car brings a bubling sound that I didn't feel on the regular engine - it sounds fantastic. The shove in the back is something that you couldn't really experience at the PDE - or you'd rear end the lead car. But after about 10 minutes, when the engine is warm enough and the opportunity arises, the acceleration is incredible. This weekend I went out with a couple of friends and he was driving an Audi S4. He just couldn't keep it up. The four wheel drive is not obvious at all - it is very difficult to feel it at all - the only time when I noticed is that I seemed to be able to get more power to the wheels earlier, coming out of the curves. Someone described the 996 4S as porky and having a lot of understeer - well, I can only say that this is a different animal. Even when pushed hard it feels very neutral.
To top it off, it has the curves - oh, what hips - washing the car even feels sensuous...
It is a great car and enjoy every time I have a chance to go out and drive it.
Enjoy. Envious of the X51. 
