I do really like the idea of the "font playground" in X8. Problem is, I have too many fonts.... I'll never have time to categorize them all manually. Which sucks because I would say this is probably the selling point for upgrading.
Like Bobby H said, there are some upgraded effects that I appreciate as well, though they aren't ground breaking changes. I haven't noticed any performance problems as he describes (I have an SSD which could be the difference?).
I mostly upgraded because it's new and shiny. I like to keep up with this kind of stuff. Also, it will make transitioning to X9 smoother, and on and on. Whereas jumping from X5 to X9 for example, might be a bit overwhelming.