My biggest annoyance with old iPhone software was that you could only sync to one exchange server at a time. I needed to sync with 2 exchange servers, one for my work place and the other for my Google Calendar.

My temporary fix was to use Google Calendar Sync on my work PC to sync my google calendar and my work calendar. Then sync my iPhone with Google Calendar and Contacts using Google Mobile.

My problems:

1. I like to keep my work and personal calendar separate. When I accept meeting invites I wanted it to come from my work email - not my Gmail account.
2. Google Contacts suck. I never really needed them and the sync is iffy with Address Book. (I use MobileMe for Contacts.)
3. I wanted to view my work email on my iPhone - the web interface worked with Safari - but I wanted push notifications.
4. If my work PC was ever turned off while I was away (ie. Vacation, or Microsoft Software Update) my calendar wouldn’t sync to Google - so I wouldn’t know any upcoming meetings or deadlines.

But now- with iPhone 3.0 software I can now sync with caldav accounts. I now use my one exchange sync for my work account - and view and edit my Google Calendar via CalDAV.

Instructions on how to set it up are available here. :)