Unbelievably productive. I don't think there is another good choice right now. I've been on every other framework (large, complex backbone apps, companies you've heard of) and the codebases have been insane. New programmer onramp time is down a lot, time to feature is down a lot. The codebase complexity seems to scale linearly with actions and stores, other frameworks felt more like quadratic (jk, don't respond to that).