Slackware is simple, beautiful and elegant but no more than it has to be. It just works and it leaves all the power in the world at my hands. I can make it do my bidding fast as lightning and it never gets in my way. Once you begin to get the basic principles of slack, it very closely follows the principle of least surprise.
That's not to say that distros like Ubuntu don't have their place, but there's so much automation going on in these "modern" package management systems that you really aren't going to learn anything. Sure, you'll end up with a very nice, very functional computer, but that (IMHO) isn't the point of Linux. I'd go so far as to say that I feel like Ubuntu is no longer "Linux", just like Mac OS X is no longer "BSD". Based on? Yes. Is? No, Slackware is Linux.
For people who work with Linux for a living, like system administration, you want your Linux distro to absolutely minimize the amount of work needed. When your server-park scales, manually having to fiddle around 200 Slackware installations simply isn't an option.
dealing with thousands of servers takes quite a bit of extra scripting, no matter what package management tools you use. I've used both slackware and RHEL, and while RHEL is super easy to manage when nothing goes wrong, slackware is a lot easier to fix when something does break.
Unlike ubuntu the other day, which felt like installing a mail server along with docbooks.
That's not true. You can automatically keep synced up.
Only real use I see for it is using it, once, to fiddle around and learn how Linux actually works. After that you just wants something which works and move on to Debian, Fedora or whatever.
Also obligatory demotivator link: http://invalid.ed.ntnu.no/~jostein/qupload/files/slackware.j...