Mainly I use it as a reference to look up what a certain node in the material editor does for example, or how to use a tool. The new tooltips in the editor are great for that, just hold Ctrl+Alt to read the description and there is a link to the specific documentation on that node/tool.
If I go looking for it myself I usually just use the search bar on the main page. It’s quick and easy most of the time (so long as I know what it’s called). Sometimes I will just start clicking through the links to find a certain topic that is more general, something like Blueprint Communication. I know my way around the pages pretty well and don’t have many issues with it, aside from the programming section… Still figuring that one out. 
The side bar showed us the other day will probably end up being the ideal method in the near future. I prefer to be able to expand categories to find a topic (like the MSDN Documentation for example) that remains open while searching for the info I need, allowing me to quickly go up or down a level without going back. Really looking forward to that!