First, not all browsers support it. For example, no version of Firefox supports it at this time (see: https://bugzilla.mozilla.org/show_bug.cgi?id=360420).
Second, OCSP stapling was originally conceived for only the end entity cert. This means that if the browser wants to check intermediates for revocation that payload can't be stapled, but will still require an additional check. There are several in progress proposals (spearheaded by Opera) to resolve this, but it's not finalized yet. This negates some of the speed gain on browsers that do choose to do revocation checking with this level of rigor.
But guess what, you can join the stapling party even if you aren't a CloudFlare customer! If you're using Apache (2.3+) you can configure OCSP stapling for your own website with the SSLUseStapling directive. nginx also plans to support stapling with the 1.3 release (initial preview released early this month). You can also do stapling with IIS 7.5+ on Windows.
Edit: Others have asked what browsers currently support stapling. I believe only Opera and IE9+ (probably schannel in Win7, but possibly in Vista?) at this time. Chrome is publicly moving away from OCSP/CRL entirely in favor of aggregating that data themselves and pushing it down to clients via their own infrastructure, Firefox has that open bug, and Apple's roadmap for ocspd/securityd is obviously not known.
That's very true, thanks to Maxim Dounin from Nginx
And Ryan Hurst from GlobalSign
Making SSL faster is really important to be able to support "full https" websites, with HSTS headers and Alternate-Protocol: spdy
full disclosure, I do work for CloudFlare and on this particular feature :)
And they're launching 30 data centers (oh wait, they're deploying some machines in a DC...) and blah blah blah.
Sigh. Cloudflare is somewhat interesting, but very much still in the kiddie pool of CDNs.