I used Slackware on and off from around version 8 until 10. I loved how stable it was and how much I learned by using it, but eventually I got fed up with searching google for an application, downloading it, extracting it, configuring and then having to start the process over again for whatever missing dependency caused configure to fail.

After leaving Slackware, I used Gentoo, Arch, Debian and Ubuntu, all of which have automatic dependency resolution. After a couple of years, I've come to realize that it's not as great as I once thought. Debian/Ubuntu pulls in a lot of garbage dependencies that I don't want. And Gentoo likes to shuffle around USE flags so that *something* breaks everytime I update world (this last time it was ac3 audio playback in xine).

Now I'm thinking of coming back to Slack. But I've never used any of these package managers I've seen people talking about like slapt-get, swaret, slackbuilds, etc.

I guess what I'm asking for is opinions about them, how well are they maintained, how large are their repositories, and so on. As always, any advice is appreciated

I don't know about slapt-get and swaret (have no real interest in them), but SlackBuilds.org is great!
The repository contains hundreds of Slackbuild script and they make the best Slackware packages.
And it's up-to-date!
And even when it's not, it's usually quite easy to adjust the script to a newer version.

Welcome back to Slackware.

This might be marginally helpful:
http://slackwiki.org/Third_Party_Package_Managers

I use slackpkg from /extra to keep official pkgs updated. Slackbuilds.org has most things people will need. Building from source is always a great option. Read some of the buildscripts from Slackbuilds and it will show you how to use makepkg. I use makepkg quite often now if a new version is not available from slackbuilds or it is not on the repository. Also slackbuilds.org will tell you what deps are needed, that is handy. Slackware has a huge default library in comparison to most distros.