I used to use Thunderbird but ran into some problems with it, especially with migrating the inbox between machines. Don't know if it's better now. I'd consider trying it again if I had an easy way to get my mail into it.
I switched to Windows Mail briefly but the lack of inline spellchecker was annoying, so then I switched to Outlook.
I've not used Windows Live Mail for more than a few minutes but it looked like it'd do everything I need (except import a .PST file, heh) so I thought it was at least worth a try.
...Oh, turns out there is a PST importer for Thunderbird. I never thougth to look before now. Perhaps I'll give Thunderbird another chance!