If you're blogging platform, that would be writing more blogs, or whatever metric that guides your business growth.
If yes, then roll-it out to 50%, confirm that the metrics are moving to the same direction and by how much. You should be able to answer the question: "users who have this feature do $X more/less/faster/etc.."
You can implement this all in house or use one of the tens of A/B testing SaaS out-there.