I test ECC 256bit / ECDSA on my site at https://sslspdy.com including numbers for OS/browser compatibility as well
Browser compatibility with ECC 256 bit ECDSA
Unfortunately, ECC 256 bit SSL with ECDSA signatures isn't supported on older browsers such as WinXP's IE8 and below. You can use Google Analytics for your web property to determine your current number of WinXP related browser sessions and break them down by browser and browser versions.
For example, below Google Analytics stats show 507 total WinXP Internet Explorer browser sessions for IE <=8 were recorded out of 164,995 sessions. That's just 0.3073% of browser sessions would be negatively impacted by deploying ECC 256 bit SSL certificates with ECDSA signatures. A small number, I can personally live with
Update: according to Symantec, WinXP Chrome also doesn't support ECC 256 bit SSL with ECDSA. That would raise the negative impact quite a bit with WinXP Chrome sessions amounting to 4,025 out of 164,995 sessions = 2.439%.
also FYI, alot of non browser clients may not support ECDSA as well i.e. older versions of curl <7.36 or some web app plugins that invoke curl i.e. Wordpress and WP Plugins
example with Cloudflare Universal SSL and ECDSA usage and Wordpress issues with curl https://community.centminmod.com/threads/wp-cron-ssl-connect-error-with-cloudflare-ssl-ecdsa.2276/
alot of server monitoring systems would have some type of client communicating with your server and some of those monitoring systems may not be able to connect if the client doesn't support ECDSA too !