But, I have to admit, I don't love it. I would love to have an integrated environment where client/server doesn't matter, but I don't think this gets far enough. Working in a spread of html.erb with special tags plus some Javascript doesn't feel great to me compared to React.
I like their TodoMVC app, since that gives a simple, minimal starting point. Getting persistence in that few lines of code is nice.
However, it has also at least one bug compared to canonical TodoMVC. I think the problem of this kind of too-clever binding is that it makes this kind of bug easy to introduce; whereas something with clear one-way data-binding like React, for all it can be verbose and annoying, tends to make it easier to get this sort of thing right.
(The bug? Click an existing todo to edit it, start typing, then press escape. The edit is meant to be discarded, but instead you'll see a flash of the old value and then the new value is persisted. It's not the worst bug ever, but shows that even in a simple app, experts can subtly screw up important behaviour.)