I use it to the extent that I need to in order to follow things like Jeff Richter's book, and sometimes blog posting by the like of Eric Lippert. To that extent it is important to me so I have a good understanding of the platform as a whole.
I have (almost) never used IL directly in my "day job". However, if tools like Reflector did not exist I might have to, but thankfully the tools exist so I don't need to.
Edit - the comment below reminded me that on occassion it may be neccessary to debug in IL if you do not have source code available. I can only recal doing this once but it's certainly a scenario where some knowledge of IL is useful.