TLDR; If you are new to IntelliJ (like me), use "Cmd+Shift+A" to find your way around.
Recently I started developing an application in Scala. I am an avid Eclipse user, but unfortunately I felt that the support for Scala is a bit clunky as of today. It was driving me nuts to see some of the Java classes highlighted in red as unknown classes, but restarting the Eclipse or refreshing project was removing those errors.
Based on my research, I felt that IntelliJ IDEA has better support for Scala. So I wanted to give it a spin. So far I feel so good that I decided to give it a try.
I prefer to get things done using keyboard shortcuts, touching the mouse only for absolutely essential needs. The most important keyboard (and in my opinion the most awesome) shortcut that you should know in IntelliJ is "Cmd+Shift+A". This will show you an input box where you can enter a partial string and you will be presented with the list of action or options that is relevant to the search term. For e.g. you can enter "override" and presented with actions relevant to "overriding".
For Emacs users, this is very similar to "apropos" command. For Eclipse users, this is similar to "Cmd+Shift+L", but only better.