Endnote 20, for me at least, is a HUGE disappointment. They took an excellent program and gutted all the best features. You cannot have more than one reference open at a time (no windows nor tabs for that); you cannot preview the PDF next to the reference or in the same window; and the interface itself looks amateurish at best. It really is just sad.
I do not recommend upgrading. Stick with X9. In fact, it may be the last Endnote version I ever use.