I would like to confirm that Constant Contact is a reputable company that has been around for a long time. They are in fact pioneers in email list management and marketing, one of the first companies in the field. They are not going to abuse data or spam you (willingly). That would be too ironic since they were one of the first companies implementing double opt-in policies etc.
I used to work with them more than 12 years ago. I'm not exactly a client anymore, but I have considered them recently for a project. They are veterans and can generally be trusted.