It wasn't my intention to stir up a Hornet's nest here but the OP asked is IS overrated?
In my opinion and experience IS/OS etc hinder photography these days with modern cmeras. It was a great feature when introduced but is of far less relevance today to the extent that I don't want it.
Lets just agree to disagree. I have gained a significant improvement in the quality and keeper rate of my images by disabling IS. I would just like others to benefit from this. Could I benefit even more by removing IS all together? I think so but that option is not available to me.
Just try the OFF position - you might like it..........
Myself, I tried it for 8 years with my 400 5.6 L.