I just got one in July. It's a dark green Sahara 2-door 6-spd... worlds better than the old TJ Sport 5-spd. I've still got the TJ, so I've driven them back to back a lot over the past couple months.
I'm not an off-road guy, so I'm comparing them strictly by on-road performance. Th JK is so much more civilized and sure-footed in all situations; cornering, braking (the rear discs make a huge difference) and at highway speeds it feels almost like an SUV. The power delivery, while not breathtaking, is much smoother.
One gripe I had about the old TJ was it's lack off passing grunt on the highway. The JK seems to have semi-solved this, as it's gearing seems more suited to faster driving. I am also looking foreward to driving it in the snow, since the JK has two key safety features the TJ lacked... ABS and stability control. I'm convinced both of these have already saved my asss from hitting a moose at 120km/h... it was very close, and I don't want to know what would have happend if I was driving the TJ.
I can't tell you how nice it is to have power windows and keyless entry... A/C and cruise are a bonus. Seats are much more comfortable, but the rear head-rests make for terrible blind spots (I removed them).
It's not all fun and games though, one thing about the JK really bothers me. In the old TJ the front passenger seat would slide forward to allow access for rear passengers, but for some strange reason, the JK's does not. This makes for a lot of swearing when you try and climb in the back seat.
Over-all it is a much more civilized vehicle, but if I was taking it off-road on a regular basis, I'm not sure which would be the better Jeep.