HTTP: Stateless, simple, plain-text, open, extendable. One request, one resource. Etc.
SPDY: Not-stateless, not-plaintext, includes complecting factors like prioritization and multiplexing. I say that is several orders of magnitudes of increased complexity and uncertainty baked into a protocol.
Result: Much less than an order of magnitude in improved response-time.
I say SPDY comes at such a cost that it cannot be considered worth it. Especially when you consider it comes at the added cost of handing over control of one of internet's main protocol's to a single company to be developed behind closed doors, such as Google is already doing with SPDY.
I'm flabbegastered that people aren't reacting to this. Had Microsoft being doing anything like this, people would be calling their senators to start investigations. But with Google it is evidently all cool.
You'd think people would remember the cost of handing over the internet to one single company, when the results of last time we did that is still plaguing us today (IE).
As for source, this one should do: http://www.theverge.com/2012/5/3/2995881/google-spdy-speed-t...