I'm liking Thunderbird. Ditching Windows Desktop Search and installing Google Desktop Search has worked well — suits my filing system. Well, except for the occasional __GD_something_or_other process that wants to keep running when I'm shutting down the PC.
Things I've had to get used to in the switch from Outlook:
Alt-S to Send doesn't work. Alt-Enter does (Outlook supports that too.)
The column sorting icons being upside-down.
It defaults to sending from the account you're looking at when you start the new mail, rather than a fixed default. Easily changed if you remember to check it.
It also inserts the signature automatically when you change the From account, which is neat.
It didn't take long to get used to the vastly better IMAP performance in Thunderbird.
I don't use a Calendar plugin. Tony pointed me to a Nokia phone sync, but I haven't tried it yet — I do backup my phone contacts, but for most of them I don't have email details, so syncing is not really a priority for me.
That's about all at the moment. I've imported all my old Outlook folders into Thunderbird, which took ages, but works fine. So, byebye Outlook!