I would suggest starting small. When beginning your next small project, do it in DEVONthink and play with some of the features. Explore a little more whenever you work with it. DEVONthink 3 is free while in beta so it’s a great opportunity.
A good starting place is to use it as a “Finder replacement”. I keep a lot of Shortcuts “resources”—say, text used as templates for new notes—in DEVONthink. The Universal Unique Identifier links for those files mean that I can always trust that the item will be accurately opened when I run a shortcut no matter what device. This also helped me when I recently studied for a big exam. I used digital flash cards (Anki), pointing to the exact page of relevant PDFs for each flash card. The concept I was looking for was always a click away.
Another option: If you’re doing a lit review, Custom Metadata, annotations, and Table of Contents files might really help you out.
The Manual and Help guides (in the menubar) are really thorough. Most questions have probably already been asked on the forums, too.
Frankly, the worst thing about the product for me is that it always feels like I’m not using the most of it… constant experimentation is necessary!