Well, the better screen, Clamshell FF, and long battery life were more than sufficient for me to upgrade my Z.
I have a 5000 and a 5500, and neat as they were, and as much as I could do with them, they didn't hold a candle to my Palm for day to day use due to the pathetic battery life. They also fell short on the portable computer side due to the screen (bad contrast, low resolution - trying to use the console was painful), and lack of CPU power (for movies, etc).
Fast forward several months and I use the 860 - mind blowing screen, 8hrs battery life, fast CPU, and the multimedia remote.
I ended up ditching the Palm, my mp3 player, and my laptop simultaneously. Because I use it so heavily I plan to pick up another as backup. (If you use your PDA to run your life, ALWAYS have a backup strategy. ) Since the 3000 has come out I'll get one of those instead and the 860 will be my backup and experimental platform (That 4gigs of space without tying up the SD slot is *really* tempting. )