I used to use a Sony N710C Clie but upgraded a while ago to a NX60 Clie.  I was all set to get a UX50 (mainly for the Bluetooth capability). I tried one at the store and was disappointed to see how small the screen was.  My NX60 has a larger screen and it is very sharp.  I bought a   PEGA-MSB1 (memory stick/bluetooth module) which works great with a couple of software upgrades on the Clie. I also got a PEGA-WL110 LAN Card which fits into a different slot on the NX60. These give me a lot of flexability with a better display.

I should have said the NX60 gives me a lot of FLEXIBILITY (vs flexAbility).

RE “I have been looking at Sony PDAs for my next one. The only problem is that I can’t find any new PDA with the one single function that I need the most being able to write directly onto a monthly overview of the calendar, like in “graffiti” mode.”

I would check Handango.com for software that might let you write in the calendar.  I think the software is more critical than the PDA for a function like you are requesting.