For me I started out with Red Hat 9 and then Mandrake but rarely used either of them. (Surprisingly I couldn't get either of those to detect my sound card correctly or something) and they were slow. Anyway when I switched to Slackware I still didn't know anything about Linux, (I remember one night I was trying to watch a movie in slackware and I put a dvd in and mounted it but it was the 'extras disk' not the 'play disk', then I couldn't figure out how to unmount it :/ ) Anyway, using slackware made me love linux because I started to learn how things were done. It was actually easier for me to learn Slackware than to learn RH or Mandrake because there was just one (correct :P ) way to do things.
I used slackware for quite a while and went from never using Linux to having Linux as my Primary OS. Anyway my slack install crashed and burned one night. (still know know why) So I decided to try something else and I switched to Gentoo about a year ago and haven't turned back.
I think Slackware was a wonderful learning distro but I'm a bit of a bleeding edge kind of person (running firefox alpha 2 and such) and the slackware package repository didn't get updated enough for my liking. I'm also one of those guy who likes to try things. Yes, I have KDE, Gnome, XFCE, Enlightenment 16 and 17, and about 8-10 other window managers installed even though all I ever use is TreeWM
. Slackware made me work to much to get new programs installed. I'm a big fan of dependency tracking because I would rather spend my time playing with programs instead of trying to get them installed. Mixing small self compiled programs and packaged programs never seemed to work as well in slackware either.
Configuring a system with Gentoo is about the same level as Slackware if not even lower since it doesn't come with adduser scripts or anything like that. I was also happy that I got to learn more about my system by working with the compiler and going through that wonderfully text based install.