Chrome has had SPDY enabled for at least a year now -- since around Chrome 10, I think? And according to Statcounter measurements for this month to date [1], 28.75% of all page views currently come from Chrome 19.
From the same Statcounter numbers, 17.27% of all page views are from Firefox 12. So that's over 46% of page views this month using the latest versions of Chrome and Firefox. And another few percent are just one or two versions behind. So a couple of months from now, 50% of page views measured by Statcounter should come from SPDY-enabled browsers, especially if IE marketshare keeps shrinking rapidly.
Also interesting: according to that graph, as of June 2012, Firefox's major stable release at last surpassed IE's major stable release--just a few months shy of the 10-year anniversary of the release of Firefox 0.1.
EDIT: Ha, actually, drilling back further, looks like that's not true at all: Firefox's major release has surpassed IE9 for a while now. I was misled by IE9's slow adoption curve.
Apparently not. There's an ever growing tail of users who get stuck with an outdated version [1]. It's not as bad as Firefox yet but surprising nonetheless given how much effort Google put into Chrome's update system.
I have personally experienced and seen Chrome get “stuck” and kind of just fail to update itself until a manual re-download of the installer is performed. It does usually work, but sometimes just stops. For example, I recently saw a bug report from someone who thought they were on the most recent version of Chrome — 10. (It’s currently at 19 or so.)
Well, about 12% of Chrome page views are from old versions so "almost no users" is a bit of a myth or exaggeration. But it's definitely a smaller proportion than other browsers.
Firefox also auto-updates, but has a longer tail of users from old versions since it's been around longer and had a more obtrusive update process until recently. Currently about 32% of Firefox page views come from old versions, but this is decreasing steadily now that Firefox 3.6 users are being auto-updated to the current release channel, and now that updates are more "silent."
Once SPDYv3 is enabled in a release channel the Chrome installations which only support an outdated SPDY version will rise suddenly if the current update trend is not reversed in a future version.
