I have the SL5500 running the Sharp 3.10 ROM. It is customized to my exact needs. The Z has become indispensible in my daily life. Since it is required to run perfectly every time, I don't risk it by flashing or playing with it anymore. I really want a second Z to play with (ie trying out various ROMs, beta or even alpha software, different PIMs, etc). I just can't risk it. Hopefully around Christmas, I can afford a second Z, but until then. . .
I like the rock solid platform offered by the Z. I also have a iPaq 2215 that I got to use at work, but it is not as capable as the Z. Hence the Z being my work PDA and the iPaq being an expensive (and less capable) toy. I keep praying that the open embedded or familiar projects will finally get the 2215 working. Until then (or until I get another Z), I will stick with the working model I have now. The best feature of the Z line is the flexibility of the OS. I have plans for a custom ROM and have partially completed it, but will not be installing it (or even finishing it) until I have a second Z to expiroment with.
Good luck with your Z adventures. It is the best PDA out of the box of any line out there. I am still lusting after an 860, but the wife and kids seem to believe that unimportant stuff like food, shelter, and clothes should take precedence . Oh well.