Slackware is the distribution of Linux I have been using for roughly seven years now. I use it both on the Desktop and in a server environment without any major issues with stability or security. I love the Keep It Simple philosophy of Slackware amongst many other things. If your looking for a lightweight, completely customizable, flexible, and mature distribution, Slackware is the definitely way to go. Here is why I think so:

1. Simplicity. You can start from a base and build upon it. Most other distributions you start with a bunch of bulky software that you have to remove after installation. I don’t feel anyone should have to work backwards.
2. Stability. Being that Slackware is the oldest maintained Linux distribution and is still kept wrapped in tight arms by the founder and maintainer Patrick Volkerding, there isn’t much chance for the project to stray away from its original intent. With many “big” distributions, there are so many programmers working on it, that it can easily stray away from the projects intended goals. The need to incorporate updates just because the version number is higher is not a practice of Mr. Volkerding, and probably has a large part to do with Slackware’s overall stability.
3. Command line. I’ve been running Linux since 1999 and when I first started, I started on the command line. In recent years, I’ve noticed most all “mainstream” distributions focus strictly on the graphical user interface, which in my opinion, gives new users a false impression of what Linux is all about. Don’t get me wrong, the command line is present in all distributions to date, but less focused on. I always tell people who are looking to venture into the Linux operating system world that if they want to use Linux, install something like Mandrivia, or SuSE, but if they want to learn Linux, install Slackware.
4. Direct Control. Slackware gives you, the system administrator, direct control over the machine without using fancy graphical user interfaces to hide anything. The ability to administrate and maintain your system is left completely up to you.
5. Independence. One of the many quarks people have with Slackware is that is offers no form of dependency checking with its package management. I myself would much rather have the independence and security of knowing that my package manager is not going to install five other packages and possibly screw something up along the way. When I install something that requires a dependent package, I am knowledgeable [and not lazy] enough to manually grab the package, and install it.
6. Community. Many communities are flushed with boatloads of users who do not really know what they are doing, but are offering advice, which is sometimes dangerous or bad, to new comers as if they were Linus Torvalds himself. What I have noticed about the Slackware community, whether it be on IRC, or a user group, is that the majority of Slackware users are mature, respectable, and very knowledgeable. I’ve never spoken to a Slackware user that wasn’t willing to help in any way they could. In the open source world, a community is what drives a project. Slackware being around since 1993 says a lot about the community.
7. Comfort. Lets face it. I’ve been using Slackware for a long time!

In conclusion I would like to state that if you are a new Linux user, lazy, or someone that has difficulty spending time reading documentation and figuring things out on your own, Slackware is probably not the best choice for you in a Linux distribution. The most important thing is that you choose a flavor that fits you, and that you are comfortable with, because above all else, you are the one who has to use it!

With that said, happy slacking!