On multiple occasions, for capturing visitor's email address, I've tested using popup opt-in vs other opt-in types (sidebar opt-in, after- and middle- post opt-in, and floating bar opt-in).
And popups win every time.
Most people's concerns with popups are, "I'm afraid of annoying my visitors".
Think about it — if they're offended because of a popup, imagine what happens when you sell them something?
You're here to do business with a small segment of your audience and not to please everyone right?
What is your experience building an email list with popups?