There is some UI lag on the retina display's when switching between spaces and sometimes scrolling. It will not ruin your experience, or it shouldn't, but you will see it. Each update makes it better and el capitan is suppose to be better than Yosemite, which is way worse than mavericks ever was.
I get lag scrolling web, scrolling in word processors (especially Word), and in UI stuff like flipping through spaces. Now that my machine has stopped indexing, I can say this is much resolved (though not fully). I haven't tried MS Office yet though. Where I still mostly see lag (and admittedly, this is where it most annoys me) is in Pages when I have my window maximized and zoomed to fit screen width. It's still jumpy here, but better than it was. At actual size it's almost fine.