Recs.
Updated
SpecsUpdate
Pros
Pro Good documentation
Although the Slackbook may seem outdated, Slackware aims for continuity. Most of the documentation that applied to stuff 5 years ago is still valid today. And because Slackware is just "pure linux", the Arch wiki can be used as an additional repository of docs, too.
Pro KISS - on a system level
Slackware (and Salix) are simple and predictable systems. Once you learn how GNU/Linux works as a system, these distro's won't have any secrets or surprises in store for you, and vice-versa. Learning these distro's means learning Linux, not tools created by Canonical .
Cons
Con Benevolent dictator for life
Because Salix is essentially Slackware, you're tied to a distro that is maintained by Patrick Volkerding. He's commonly known as "The Man", which kind of tells you all you need to know. He determines the pace and the course of the ship. As a result, you're anything but bleeding edge, but the least likely thing to happen is a system upgrade breaking something.