I work as Independent Consultant, with several customers at the same time. And each of these customers has several SharePoint farms, with different features and services. And SharePoint farm documentation is always a challenge...
First, let me mention SPDocKit's licensing and installation: there're two licensing models, you can buy it for your own farm OR you can have a consultant license (like me). In this case, you're allowed to run SPDocKit on as many farms as you want, even without installing it! For example, you can take it and run on your customer's farm and generate a full farm documentation with a few clicks in minutes. It's more than amazing!
Second, the documentation it generates.
It helps a LOT, saves a LOT of time when you have to document (and/or compare) your SharePoint environments.