Today, Google admitted on the mailing list that 90% of all Gmail and other Google services go via SPDY when the client is Chrome and SSL is used:
Yes, indeed SPDY is enabled in Chrome and on Google servers for all SSL
traffic at this point. (Actually, we do 90% on SPDY, with a 10% holdback
for purposes of A/B comparisons).
This is pretty damn significant real world usage.