An example of a sophisticated meteor app I've built to test scalability http://www.qckt.me. I've noticed some performance issues (to be fair it is running on meteor's free hosting service, however they don't set resource limits or anything like that) maybe I'll share some stats I've been collecting in the future.
Like every web framework, I've run into problems. The simplicity of the server-client reactivity is keeping me in. Its a good idea and the got the funding to keep it going.