Adding to this: Any time there's major changes to a program/method/UI, I typically find it's good to identify the 5 or so tasks that I need to do *everyday*, and learn those 5 things first. This takes the stress of something new away, as you know you're (at least) EVEN in productivity as you were before the major changes took place.
Then, take the next 5 new features and tackle 1 every day, as these other wise gentlemen have suggested.
I'm also comfortable knowing that I don't know everything about SONAR, but I know the stuff that helps me serve clients within the context of our work. Craig's tips serve as introductions to functions I would rarely use, but will often check it out after he explains it...if only to say "woah cool..had no idea SONAR could do that". ( Of course, I then promptly forget how to do that task and never do it again, but that's another thread :) )