In terms of complexity, the software I'm developing in Meteor has a lot of moving parts (animation, physics, artificial intelligence, and hardware integrations) and Meteor has handled it all tremendously well. In terms of user scale, I haven't had much experience, due to the nature of the project. It has made our development team tremendously happy, and provided a tremendous amount of flexibility compared to other approaches we've tried in the past including Objective-C and Java.