I think they totally work! The key is to offer value specifically to your audience that they actually care about. So if they arrive to your mindful steward website, have a popup explaining why they should subscribe to your email list. DON'T use something like "get updates and deals". Here's a link to some great examples of what you should do: http://blog.justuno.com/10-inspiring-ex ... -they-work
Think about if you were your audience, what's something you could give them that they would feel is worth sharing their email with you!