I've been a KDE fan since 1.x, but one of the worst pieces of OSS I've ever used was in KDE 4.7 -- KMail 2. I thought it might just be me, but a little research showed that every distro shipping it has had many angry users. For me on OpenSUSE it's been an endless source of lost incoming and outgoing mail, performance problems, and generally horrible bugs. Totally broken development process -- the problems were widely reported during at beta, but ignored since KDE leadership insists on pushing the buggy/leaky Akonadi-Nepomuk stuff regardless of what it means to end users. I'll give KMail in 4.8 another chance, but I don't hold out much hope -- it's been years since Akonadi was introduced and everything associated with it has been a disaster.
The rest of KDE 4.7 is absolutely terrific though.