Pop ups should be easy to close. Nothing is more annoying than having a pop up appear and you do not know how to close it. Sometimes your visitor may not be interested in what you have to offer and so will decline the offer by closing the pop up. If the ability to close the pop up is difficult, such as the exit link is not visible, you can lose visitors. Therefore make sure that your pop up can be easily closed to avoid losing readers.
chantal.. yes we love his posts! I compared aweber with benchmail.. both are great, i liked the added polls, and survey ability on benchmail. sign up for both, play around as they both feel different and both have a lot more added features than mail chimp, unless you are just one company, and not doing a lot then mailchimp will be ok. For me no mater how small the intention is always to grow. Good luck