1. It is extremely flexible.
2. It is available in three forms under the "antiX" name: Core, Base, and Full.
3. It is a lightweight system, and it runs well on aging systems, and extremely fast on newer systems.
Though the original antiX was developed primarily for older systems, a more recent variation, also developed by the author of antiX, known as "anticapitalista", anti, as he's often called, also has collaborated with the MEPIS Lovers community since the origination of antiX, and over the past year, he also developed another excellent distribution, MX-14, an Xfce desktop distribution that uses a few more of the MEPIS-based tools, and he developed it based on mutual interest and with considerable help and feedback from the MEPIS community.
That same community has also been working on another KDE-based variation of MX-14 that some of the original MEPIS community is experimenting with. At this point, I've not seen an official build of this work. Instead, there are some instructions and tips for those who want to build a KDE version of MX-14 directly from the Xfce-based work. I have not personally verified or checked out that work, but in the past, I've built my own KDE and Xfce custom systems from either antiX Base or Full, and I have an existing Xfce-based system that I built from the ground up with antiX Core. I even wrote a now aged tutorial describing the steps I took in building that system, and a few people have used that tutorial to build their own similar systems.
The ease in which that can be done by someone with a moderate amount of skill shows that while, on one hand, the antiX Core effort may not be the choice for a beginner, it doesn't take years of expertise to put together your own system because antiX provides so many excellent tools. The fact that the current MEPIS Lovers Community has also performed similar work adds evidence to the claim that it's reasonably straightforward to build a wide variety of very usable custom systems from any of the antiX tools found in each one of these excellent distributions.
Creating your very own antiX core system from scratch, my old tutorial, can be found on this blog at: http://brianmasinick.blogspot.com/2012/04/using-my-customized-antix-core-system.html
This article is not the tutorial itself, but it discusses the tutorial, provides links to the original tutorial, and also to additional information about antiX.
I also wrote some comments on the Everyday Linux User blog in the article entitled