Having worked on a project recently where we chose jQuery (because it was already on the page and we didn't want to delay page load), what I missed about React was the luxury of re-rendering the view from state.
Doing so with jQuery blows away the DOM (causing inputs to lose focus), so we ended up with the usual jQuery soup referencing the DOM for state.
My rule of thumb is that React is a good solution for any site that needs something more complex than simple, slideshow-level js, or a site that more than one slideshow-level widget (slideshow + order basket, etc.).
Doing so with jQuery blows away the DOM (causing inputs to lose focus), so we ended up with the usual jQuery soup referencing the DOM for state.