Learning the core basics is essential though. All the sums, countifs, some vlookups and stuff. Without an understanding of that you're going to struggle with anything more complex. Guess it depends on what you're doing. You can self teach all of the above fairly easily I'd have thought.I must admit, Google is your best friend. Some functions you might only use once or twice and you will have forgotten how to do it the next time you need it....
Just learn what you need, when you need it.
I always think learning Excel on a course is pointless - you don't really know what you need to know until you need it, and by virtue of the fact that you don't know what it can do, you might not ask the right questions.