I like this a lot. One thing I noticed is that on the video, the first change is on the background from green to red. As a deuteranomalous (color-blind) person, I really couldn't see the difference unless I watched really close while it happened (and didn't blink). A small thing, but that kind of thing can be slightly frustrating for us. At any rate, the other examples were helpful in giving a feel for the product.

On a philosophical level, I like how this should integrate well with the rest of the Unix utilities. It isn't as immediate and interactive as Bret Victor's examples, but I think this has the advantage that the code is actually there and it works. It's not the ideal system, but I suspect that it is an improvement over the status quo. I'll definitely be keeping an eye on this.