The rendering was a bit slow, and in more than a few places I had to force a refresh to get it to pick up data changes. Those are really my only complaints though: the templates were a breeze to write, and with the ability to write custom formatters, Rivets is pretty expressive and extensible. I wound up with a codebase that was quite straightforward and easy to reason about, with minimal cruft. Overall I'd say it was a win.
Nowadays I use React.js and am very happy with the switch, but I definitely recommend Rivets to anyone who just wants to add a smidgeon of data-binding without pulling in a heavy framework. If I were doing that medium-sized SPA project again though, I'd definitely use React.