You have to really change how your website gets served (at the cost to non-SPDY users) to get an increase in performance. Which usually ends up not even being 25%.
From what I've been able to gather, especially with little to no reports of any real-world benefits (I've only seen criticism of "bad" tests from SDPY supporters, but at the same time no one has posted a "good" test), I have to say SDPY is turning out to be hot air.
I'm hoping I'm wrong.