If it is truly an 'opted in' list then the emails are pretty targetted and the people are looking for the information that you intend to target them with and as such you will probably find that they are quite effective (depending on your copy writing skills)...
Usually, not all, these so called 'opted in' lists are scraper lists..ie. bot gathered or were built using incentives and the majority of sign ups used throwaway addy's.
Email marketing CAN be effective provided the list is reliable and that you email provider/host allows you to use that many mail outs...a million offers (or even 500,000) will put some strain on most servers.