They offer both now and I think the new Component integration they offer is perfect. You can see the code here: https://github.com/ultimatejs/tracker-react/blob/master/main.js. You can also see it in action here: https://github.com/thespacedojo/twitter-clone-react-master/blob/da12c1aa7a4b9dd89e3af504a117c89dd0c0c79a/client/messages.jsx.
You can see the FlashMessage.find and remove calls are just in there and everything works reactively like it should. Super happy with it so far.