I have briefly used all KDE flavours from KDE 3.0 to 4.0 to latest plasma shell with KDE 5 series. But I could not get along with KDE for too long. I use it for few months and then get bored or just annoyed enough with the way many of the things are and hop to other distro.
My main gripes are below:
1. The themes and the overall widget animations: Animations are good and bright, catchy colors are good too - if used sparingly. But with KDE it's just too much. Every time I have used KDE I ended up disabling almost all animations, and switching to an easier on the eyes color theme. The various themes just don't look natural - they look like some snazzy designers dream. Rather take a look at the Mint themes, or Ubuntu themes - they are muted and not eye catchy. But that's something makes you forget about them and concentrate on the work. With KDE it's exactly opposite. The various colors, gradients, and changes keep grabbing at your focus. Add to that the various popups and animations. You move the mouse to the window buttons on taskbar and every time a giant preview pops up. Man, that's so annoying.
2. Too much customization: The customization options offered are just too much. It takes more time to go through them all and just come to a sane combination. Rather useful defaults should be the norm. E.g. Why that giant taskbar button window preview thing on by default? Jeez!!!
3. KDE Has been RAM hungry: Previously KDE has been resource hungry. The KDE 4 series suffered from memory problems too. On many occasions I found out that I've max'ed the RAM in an hour. With plasma this situation is better, but still KDE runs so many services, it just uses more RAM and that means your 5-6 browser tabs can bog down a system low on RAM.
4. Difficult to trim down on the FAT: Since so many of these services are intertwined, it's difficult to trim down a KDE based system. That's one of my reasons to not use KDE much. Half the time you remove something critical and it breaks the system. With Ubuntu- unity, Mate etc the chances are less - in fact with Mate the system is already quite trim. Again plasma does better with this, but last time I tried trimming down a plasma based desktop - plasma neon to say - I managed to break it!!! Grrrr!!!
This is not a rant. But just some reasons why I've stayed away from KDE through the years. And it's more than 14 Years to date since I popped in a Slackware Live CD in a Single Core Pentium 1.8GHz based desktop's CD ROM drive.