Thanks so much to the team from WMDE for making this! I've already used it twice in the last few days to trawl through a page's history and figure out why particular bits of content were added; before I knew about this tool, I might simply have ignored those issues because searching the history would have been too difficult.
I do have a couple comments on the UI. First, it took me a significant amount of time to understand that you click in the lower half of the slider to set the start of the diff, and in the upper half to set the end. For a while, I just clicked somewhat randomly until I got the diff I wanted, which wasn't a great experience.
I think one of the reasons this was confusing was that the vertical axis of the "graph" is already used to indicate addition and removal of content, so I wasn't expecting it to have another meaning as well. Personally, I didn't find the graph very helpful (the edit summaries were much more useful, even though they were harder to access). Partly that may be because the graph isn't to scale; for example, in the history page I'm looking at right now, +11 bytes is about half the height of +364.
Second, when you've got one revision set as the endpoint of a diff, you can't click on the bottom half of that edit to make it the starting point of a new diff. I don't see any reason for that restriction; I would expect clicking in that bottom half to advance the starting point like normal, and advance the ending point by one as well to compensate.
So, all in all, I found the design somewhat unintuitive, but now that I've figured out how to use it, I really like it :)