In my opinion, going for blog visitors is better/easier than going after subscribers. I have a subscribe box on the right hand side of my blog's site, but I suppose one of those pop ups might be more effective. I've noticed that many websites prompt you to subscribe as soon as you get on the page.
Perhaps it's just me using subscriptionflow.com for subscription management, but I read a lot of blogs but never feel compelled to subscribe to them, and I'm sure many others do as well. I usually just come back to see if there are any new posts, so perhaps you need a mechanism to entice folks to subscribe?