I already have Diskeeper 8.0 and run it frequently, works very well. I have seen there is Diskeeper 9.0 out, and I am wondering if it's worth the purchase or not. Is there much of a difference between the two?

I have been a faithful follower of Diskeeper for many years. Switched to DK9 shortly after it came out. Upgrade is not that expensive anyway. I always wait a few months before I make the change, in case there are any glitches.
It is definitely a lot faster than DK8 and perhaps it looks a bit prettier.
I don't really know, because it runs in the background and I never see the interface anymore.