I like solutions where I don't waste too much time managing/filing email (dragging into tons of folders just starts taking too long)
I liked Opera M2 for its "one box, virtual folders for on the fly classification" approach, but it would get slow after a certain size (i have years of email) and didn't support tags the way I would like. I need to revisit it though as there are many new versions since I last used it.
I liked Outlook used in a "big mailbox then using virtual folders based on categories and filters" scenario, but Outlook just cannot handle the amount of email I have/had in certain jobs, and I don't like its storage. I used xobni to add some magic as well. But there's a limit to how much it can handle on imap without slowdown
So I currently use
postbox which is a shareware cleaned up version of Thunderbird for both mac and windows - neat features around search and threading, very nice for daily use, some clever web integrations. Compatible with a subset of Thunderbird add-ons especially gpg (which is actually how I found it). I especially love the summarized and conversations views, very handy on long work discussions.
http://postbox-inc.com/featureswhat’s new in Postbox 3
Awesome Gmail Integration
Native Gmail Label support
Dedicated view for “Important” labels
Send and Archive functions
Support for Gmail keyboard shortcuts
Convert detected dates to Google Calendar events
More Socially Connected
Profile photos from Facebook, LinkedIn, Twitter, Gravatar and the Address Book
Automatically displays job titles and company names from LinkedIn
Quick and easy access to LinkedIn, Facebook and Twitter profile pages
Update status directly to Facebook, Twitter, and LinkedIn
Works with Dropbox, Evernote, and more!
Send Dropbox links instead of the files
Send message content to Evernote on the Mac
Use Date Detection to quickly create events in iCal, Google Calendar, or the Lightning Calendar Add-on
Support for the newest version of Growl (1.3)
Canned Responses
Create a set of pre-built templates specifically tailored for message replies
Super useful for replying to common inquiries
New OS Integration Features
Support for Mac OS X Full Screen Mode and Trackpad / Mighty Mouse Gestures
Windows 7 Jump Lists and improved interoperability with the popular Minimize to Tray Add-on.