There are new models that should be over here soon (hence the discounts) one that is functionally the same but with a faster chip and one that is a very different beast altogether.
The new 5600 model is not actually as good as the 5500 (these are things I have read online BTW) , basically they have stuck a new cpu in it (400Mhz instead of 200Mhz) but due to implementation problems it will run at about the same speed as the 5500. Also they have taken the amount of RAM down to 32MB from 64MB but bolted on 64MB of non-volatile flash instead of 16MB of flash in the 5500. This means you will be limited to a maximum of 32MB of RAM where my Z is currently running with 64MB (and 128MB of MMC for its file system) with openzaurus. The main advantages of the newer model is that it has a 1700mAh battery instead of a 950mAh (but the new cpu burns more juice iirc) but this adds a bulge onto the back of the unit, and it has a built in microphone which you don't have with the 5500.
All in all it is a cracking little widget, as a basic PDA I'd be forced to say that it is not as good as a decent Palm, but as a Linux machine in your pocket it is fantastic.
I would say that it makes the palm look like a pile of something brown and smelly :) although I have now had 4 different models of palm break on me in pretty much the same way. I do find the Z is much better as a PDA. One thing is that I did have problems with openzaurus 3.0 (and the sharp rom too) but I now have openzaurus 3.2 installed and it works very nicely, apart from the occasional hot plug problem.
Oh and install Interstellar Flames to impress your inquiring friends (the 3D graphics are out of this world on a PDA)
I have this, it does tend to impress people when they see it :) although I still have to check out other games yet.
Adam