I'll admit that I've only been using GNU/Linux for about a year and a half now, but I have tried out a few different popular distributions so far. It took me a while to realise it, but I eventually realised that KDE is ultimately the best desktop environment presently. However, GNOME appears to be more popular when it comes to distro support of it (Red Hat, Ubuntu, and Gentoo come to mind), so it seems to be more difficult to find a well-done KDE distro.
I will be testing and writing mini reviews for several distros that support KDE in the hopes that I find the best KDE distro available. So far, I have used and installed Kubuntu, Debian, and openSUSE.
Debian - Sid
Debian is a solid distro, no doubt, but for desktop use, we really need to try using Debian Sid. The problem with Sid, of course, is that it literally is unstable and will break eventually. Then again, the more breakage you deal with, the better you become at Linux administration, so this could be a "feature" as well. Debian seems to have better support for up to date KDE than it does GNOME (or at least it did for a long time while Sid was stuck with GNOME 2.10 while GNOME 2.12 had been out for a while), but that may have been due to the fact that KDE releases have dramatically slowed down while KDE4 is developed. Debian has excellent package management, especially when it comes to breaking up the monolithic KDE packages (e.g. kdebase, kdenetwork, kdegames) into individual programs and libraries. I could go on and on about how much better dpkg and apt are than rpm and yum/yast/whatever, but that isn't the goal of this review. Now although Sid supports the latest and greatest stable releases of KDE, it doesn't do a whole lot of its own custom work other than proper packaging and occasional patches. Debian is good if you like vanilla KDE. Hell, if you like the old KDE releases, Debian stable might be good for you, too.
Kubuntu - Breezy, Dapper
A Debian derivative, Kubuntu is the KDE-centric part of Ubuntu. Kubuntu's goal is to be the best KDE distro available, and they sure do a good job at it. Kubuntu adds in a few of its own new KDE packages (such as KDE System Settings, a Mac OS X-like system preferences interface, replaces KControl, the KDE Control Centre), and it offers pretty up to date KDE as well. The difference, however, is that although Kubuntu is quite up to date when it comes to packages, it does not suffer the instabilities of Debian Sid. During Ubuntu/Kubuntu/Xubuntu/Edubuntu/Jubuntu/etcubuntu development, Ubuntu gets most of its package updates straight from Debian Sid, although it sometimes will test something (e.g. Xorg 7.0, the modular X11) before Sid gets it. Kubuntu is a solid KDE distro, and it is my favourite so far. Don't forget that you get the pure awesomeness of apt as well as a few pretty interfaces for it (Adept is the default GUI apt program).
openSUSE - 10.1
OpenSUSE is the "community edition" of Novell's SuSE Linux distribution, and many praise it as the best KDE distro available. However, I would have to disagree. SuSE aims at being the most simple and easy to use Linux distro available, and that apparently translates to "make it as similar to Microsoft Windows as possible without the viruses/infections/spyware/trojans/adware (V/I/S/T/A)". I'm completely serious; the entire installation is done in a Qt environment, and the default desktop even contains a "My Computer" icon (although it goes to sysinfo:/ in Konqueror which is much cooler and more useful than Windows' "My Computer"). The package updater (which is based on rpm, mind you, an annoyance in itself) even looks and works like Windows Update. Since SuSE is an rpm-based distro, it is much more cumbersome to upgrade the entire distro without burning the new ISO(s) again. With that in mind, many packages are also frozen in the stable release, but you still get security updates for 2 years after the next version is released. I'd only recommend this distro if you seriously like the Windows interface. Although, SuSE does come with a lot of packages on the install CDs/DVD, but Debian and Kubuntu have like 20000 packages anyways. SuSE is certainly a bit more "n00b-friendly" than Kubuntu (which is definitely saying something considering how easy Ubuntu and Kubuntu are to use); I'd even call it your "grandmother's Linux distro", but I hate that comparison as my grandmother uses OpenBSD 3.9 (not even I can install that without buying the CD).
I plan on trying out FreeBSD, Gentoo, Fedora, OpenSolaris, and possibly some other major Linux, BSD, and *nix distros to see how well KDE is done. However, at this point, I'd recommend Kubuntu if you want a KDE/GNU/Linux distro. Who knows, maybe a BSD will turn out having better support?