An great overview and introduction to my favorite CMS, by Francesco on Smashing Magazine:

I think what resonates with a lot of people is that ProcessWire is a system that goes from simple to complex, not the other way around. It doesn’t assume what you want to build, but instead lays a strong, non-opinionated foundation by offering you effective, powerful tools and leaving the rest to you. That conceptual aesthetic has, to me, a certain appeal to it.

I like the term "non-opinionated content management" to describe how ProcessWire will not make any assumptions on how you will display and connect the data. This is why people starting with ProcessWire will have some difficulties figuring out how to start. There is no right or wrong way to build with ProcessWire - and this is why I think that this is no beginner-level CMS, well, it even isn't a CMS per se, but a CMF - a content management framework, a framework that helps you to build your own CMS, tailored to exactly what the current project needs. So you need a clear idea of what you want to build, and which data you need to manage, and for that, you need some experience.