KDE 4 happened
I used to enjoy using KDE but I feel they lost their way and are overly attracted by "shiny new things" like workspaces, activities, or semantic desktop - virtually everything being dependent on strigi then akonadi then nepomuk then baloo or ... The real bugbear is that every time they ditch everything for a new metaphor - so much of the previously working and reliable software is also ditched or obsolete.
It doesn't suit my needs to have to keep re-configuring my computer and software and workflow so I stopped using it.
I do think it's great and valuable that it's there as an option though.
$ free; akonadictl stop; sleep 15; free
total used free shared buff/cache available
Mem: 3934720 1227872 1296492 253868 1410356 2203068
Swap: 0 0 0
total used free shared buff/cache available
Mem: 3934720 897308 1626924 249452 1410488 2538072
Swap: 0 0 0
I'm still sticking with KMail for now, though. The UI is a bit nicer (esp. w.r.t. GPG), and mail checking as a background service is nice.