I've been using KO/PI, KA/PI, the password manager, and KT/PI (time tracker) on the Z 6k. It's been great. I had used the apps on a 5500, but the screen space made it a little cramped and hard to see things. The larger resolution on the 6k (and the fact that you can reliably rotate the screen when you want to) makes it a solid application.
I tried using the email client (OM/PI) and didn't have much luck with it for mulitple email accounts. QTMail seems to be a better app for my purposes.
~Ceffyl