This is a good article covering some of the pros and cons of using a pop-up form on your blog. There are some really good points made for both sides, like pop-ups are annoying and can cause readers to leave your site. But if you are looking to build an email list, people are blind to the opt-in form usually found in the right column and therefore a pop-up helps. You be the judge.
Read the full article here:
Should You Have A Pop-up Opting Form On Your Blog?