Works brilliantly - low memory usage as well for both platforms. (We tested all the way down to old Android phones and still nice and smooth!)
We've just started using it at Memrise and it works like a dream. The iOS devs are loving us at the moment.
I have played around with it a bit with one of our devs. I haven't gotten into much depth with it past a simple looping animation or two (which worked beautifully). Prior to Lottie we were using PNG arrays which displayed fine, but were extremely limited programmatically. Lottie animations provide much more flexibility in terms of responding to various interactions on the screen.