I was one of those bitten by the upgrade to KMail back in October. After spending nearly two days without access to email while trying to get the upgrade and migration to work, I gave up and pinned KMail to version 4.4.11.1.
With the recent upgrade to KDE, I'm curious if anyone knows whether the bugs have been worked out and the KMail upgrade will go smoother now? -- "Marriage, n. A community consisting of a master, a mistress, and two slaves, making in all two." - Ambrose Bierce