A few weeks on I've settled on the following:
1. Calendar:
1.1 Google calender for events
1.2 Calendar pad (and widget) for viewing my diary.
2. To do's:
2.1 'Thinking space' mind mapper for listing my tasks (I like mindmapping- this way I can do "tasks" by week, by month etc and hide the detail if I want).
2.2 Astrid for very specific to-dos
2.3 I still have 'Pure calender' (and widgets) which allows me to show tasks and to-dos by date. I'm using this a bit less now.
3. Notes:
3.1 I'm using 'Thinking space' mind mapper for things like shopping lists etc (mind maps are great for flexibility. It looks like you can link/sync maps on your andoid to maps on your PC through 'Thinking spaces' cloudify option. This uses google's servers- though I've only done a bit of basic syncing of maps between my phone and pc).
3.2 '3banana' for adhoc notes (I can save details from apps to 3banana directly). It's simple and effective.
3.3 I'm half using Evernote still. But the implementation on android could do with some imrovements (eg I don't like having to hit the menu key to get to a list of notes). I heard that Evernote are working on improving the android implementation so definitely one to watch, especially as the PC version is so great.
(Also, using dropbox for syncing files. 'More icons' has helped a lot- it allows 4 apps to fit into the space that would normally have one app. That frees up some of my HTC Desire screens to have larger widgets, like my monthly calendar).