I had used Slackware as my primary and only OS for about 6-8 months or more in 2005-2006 - it was Slackware 9.1 to 10.2. It was a system I had complete control over, and knew inside out - I think I edited almost every config file, including the minimalistic bsd style init script - I remember there were just one or two init scripts that contained all the services and I had added more messages and color to make it look like Suse or Debian, and also added kdm to it. I also compiled my first own minimal yet fully functional Linux kernel on it . There was no package manager, no dependency tracking. At times it was a boon - programs would run with missing libraries, just some functionality missing here and there, which mostly didn't bother me. For those interested, I got a lot of 3rd party packages from http://linuxpackages.net/ (meant specifically for Slackware, but untarring the tgz packages should essentially work on almost any similar Linux distro). This, together with the Slax live cd that is based on it, was a great learning experience - most importantly, it almost brought an end to my distro hopping, and taught me the patience to deal with issues and understand and use almost any Linux system.
Slackware was my last-but-one distro. I now use Gentoo. Here is my timeline again:
mid 2003 to mid 2004: Red Hat 8.1-9.0, Fedora Core 1, Mandrake 8.1 - 10.1
mid 2004 to mid 2005: Debian testing/sarge (started to "learn" and appreciate Linux)
mid 2005 to early 2006: Slackware
I should also mention that in late 2008 - early 2009, I found a little less time for Gentoo and being a KDE user, found 4.0 very rough, so I was using Ubuntu/Gnome as my primary OS for few months. I ended up touching almost all config files and had a good understanding of it, thanks to patience and experience from Gentoo. Also I felt Ubuntu 8.04.1 (not even 8.04 original) was the first Ubuntu that worked really well and really well for both new users and power users like me.
On that note, congratulations to both Slackware and Ubuntu for the great releases and thanks to them for helping me in their own way. Oh, and Gentoo FTW .
Well, long live Slack.
Being a faithfull user since 2002, I use it on all my systems including laptops. Simple to use, close to upstream, very quickly set up to my taste in usually less than 2 hours (and I mean a full desktop), providing a full and consistent development environment, allowing for easy compiling of its own package without any fear of breaking the package system.
Just great stuff.
Congrats and many thanks to Volkerding, Workman, Hameleers, Piter Punk and co.