Tried Microsoft Entourage - didn't like it - it's nothing like outlook and it's UI is as if it's been resurrected from 1970s. And without exchange server to connect with, it doesn't have much advantage compared to others.
I tried Apple mail also for a couple of months. Didn't like it either. Although it looks great compared to entourage or thunderbird, it isn't great for handling lots of e-mails in lots of IMAP folders. It's search is also lacking in speed.
Then I tried Thunderbird. I've been a big fan of Mozilla for a long time. And being a supporter of open-source (where appropriate!), I decided that I could put up with minor quirks here and there with Thunderbird and woud use it as my primary mail client. And so I've been using it for past 2 years.
Over the years, Thunderbird has improved quite significantly. Especially it's ability to handle huge number of e-mails in huge number IMAP folders is great. It's search is also quite fast. Although there have been lots of crashes (as I'm always on beta or even alpha builds, that's expected), the latest Thunderbird 3 release has been quite stable. No crashes so far. So overall I'm happy.
But I think thunderbird can do much better with just a few minor improvements. Here's my list of low-hanging-fruit enhancements to thunderbird that can greatly improve it's UX.
- Keyboard accelerator or special keywords (search operators) that maps to search filters in the quick search drop down. This would speed up the search experience in a big way. I've filed this as a enhancement request in the thunderbird bug tracking. https://bugzilla.mozilla.org/show_bug.cgi?id=538738. Please leave your comment there if you also think it's important.
- Multiple addresses in a single line in the compose window. This is annoying when we are replying to a message having a lot of recipients. Here's the enhancement request for this one. https://bugzilla.mozilla.org/show_bug.cgi?id=495241
- In thread view, when a new message arrives, if the thread is collapsed, it should be shown in bold to indicate there is a unread message hidden there. Otherwise, the user may miss reading the message.