I’m using KMail 1.9.50, and I love it!
There’s still the odd quirk hanging around, such as not saving expire settings, but on the whole it’s a pleasant experience. And I love the Oxygen theme.
There are various improvements over KMail from KDE 3, as well. For example, threading is specified on a per-folder basis – so you can disable threading on lists where people abuse it (such as my college mailing list, where people send emails to the list by using reply-all on a previous email) or where you don’t want to look for emails by thread but by date (such as sent-mail), but enable it on something like kde-core-devel.
It has a very weird interaction with the plasma system tray, though. Having started KMail, sorted through a bunch of emails and quit it again, my system tray looks like this: