I use Cinnamon because it doesn't randomly drop indicators from the panel like Gnome Shell did for me, and it's easier for me to use than Unity. It's also plenty fast on my system, and has no apparent bugs. I have found some bugs, but I had to go out of my way to replicate them, rather than seeing them in daily usage. I had several bugs that appeared in normal usage when I was running Gnome Shell.
Also, Unity plain upsets me with its dock. I greatly dislike it as I always seem to click the wrong icon, or it doesn't scroll when I need it to, or some other stupid little thing. I did use it for a few weeks while I was waiting for Mint 13. I believe I will stick with cinnamon from now on. <bad pun>
stick, cinnamon, cinnamon stick! Pardon me while I try to sober up.</bad pun>
I don't run KDE anymore for consistency across my systems. It was fine when I was running it, but when the motherboard and HDD failed in that system, I never reinstalled it. The KDE applications and configuration methods weren't my cup of tea (but I do miss having an apply button), and it was too much to remember between 4 different desktop environments/operating systems.
I recommend Mint with Cinnamon to people who are switching to Linux because it is close enough to Windows for them to get their feet wet, while still being customizable enough for them to get their hands dirty.