Any tips on integrating existing Mailchimp newsletter with Membership?

I have an existing Mailchimp RSS feed newsletter and database of users. I would like any advice on best practices integrating it with Membership features.

For instance, any existing or new newsletter signee should equally receive any new post that I put on my WP site, but should I keep that database on Mailchimp or migrate it to Membership? Are there any cool Mailchimp features (such as a custom HTML newsletter format) that I will lose if I do?

However, I don't want to maintain two databases or have to enter new members from Membership by hand to the old database--or gasp--from Mailchimp to Membership.

BTW Does Membership offer a RSS feed option for new blog (newsletter) content? If so, how does that content look in the user's mailbox?

I also plan on protecting some paragraphs of content for newsletter subscribers, keeping it from unpaid users.

What are others doing?