My wife has the original Desire and I the Z. It might be just me trying to justify my purchase but my Z feels more responsive than the original Desire in use. As many have already said on the net, the cpu on the Z is next generation on from the original and the clock speeds are not comparable - it's like comparing a desktop cpu from two generations, the newer will be able to do more per clock cycle. Also I heard the GPU in the Z is far superior than the original, but I haven't research into that myself.
If you can afford it, get the Z. I'd imagine HTC will stop upgrading the original Desire before they would stop the Z & HD.
* The Z is a lot weightier than the original.
* I can't tell the difference between the screens, both look the same to me
* The music app on the Z can stream from upnp media servers, the original cannot even after Froyo update.
* I was apprehensive about the touch menu buttons instead of the hardware keys, but in use, I much prefer it than the old hardware buttons. A gentle tap on them is all that's needed to access the home, menu, back & search "buttons", makes the experience more in tune with the rest of the touch screen experience.
* The touch pad is bit rubbish. Not responsive at all, I struggle trying to use it to move text cursors around text boxes. Definitely the weakest link of the Z experience.