I've worked with a lot of linux distros. I'm a Fedora person myself, but enjoyed Debian and Gentoo also. For Slackware heads, why should I look at Slackware?
Speaking of patching, Slackware tries to stay as close as possible to the upstream developer's version of a package, with minimal patching. Patches are usually security related, and sometimes a package is patched to fix a bug found in testing, but there are no exotic tweaks like what you get with Ubuntu and other "flashy" distros.
Package management is as simple as it gets; there is no dependency resolution so the user is responsible for making sure they have everything they need for a particular package. Personally I prefer this approach, it reduces bloat and puts me in control of my system. That said, it's not for everyone, and there are some third-party package managers that enhance the existing system with dependency resolution and a pretty interface. You can also build from source just about any package you want to, as Slackware includes a complete dev toolchain for the most popular languages, and you can easily install support for less popular ones.
There is no systemd, which to me is a blessing as it makes for much easier and more familiar management.
Obviously I'm a fan, but there are some downsides I'm willing to admit. It's not flashy; the various desktops that come with it (KDE, Xfce, Fluxbox, Blackbox, WindowMaker, fvwm, twm) are bone-stock with no customization like you get with other OSes. This means a ton of tweaking on the user's part if you don't care for the default themes.
GNOME support is nil. Pat dropped GNOME in the mid-2000s when he got frustrated with building it, and while there are third-party solutions, none of them work 100% in my testing.
Unless you run -current, the stable version tends to get left behind. It's been nearly three years since 14.1 was released, and it was clearly showing its age, though some would consider that a good thing (I certainly do, especially for servers, which is what I use Slackware for on two systems).
There is no systemd, and for many people that makes it a non-starter.
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I say, give it a whirl. You may be pleasantly surprised! Then again, you may run screaming back to your bells-and-whistles eye candy OS. But at least you can say you tried it. :-)