I remember reading their blog post about their "Holy Grail" framework, but I couldn't really understand what made it so cool. Can someone explain to a noob like me why someone would consider Rendr the Holy Grail of frameworks?
So you still need both backbone & mongoose/whatever validations if you want them both on the client & server side. In practice, the system is fast enough that server side validations may be sufficient.
Given my experience with the very similar highbrow, I expect that rendr can return a fully functional page in 10-20 ms. In a typical client-side JS framework, the first page takes 300-3000 ms to render. Subsequent pages are lightning quick, but in many cases it's the first page that's the important one.