Hi Kimi,
Congrats on moving to SFDC. (so much nicer there, like going to the beach)
Data.com has issues with the integration. See my full guide here:
http://www.marketingrockstarguides.com/how-to-use-data-com-with-marketo-269/
What you decide is up to what you want to accomplish:
-Account info updates? (company, SIC codes, addresses, hierarchy)
-Missing lead info updates? Can be helpful with non-MQLs
-prospecting - buy your team LinkedIn passes
My feeling is to stick with the SFDC version and avoid integrating it with Marketo. Or try Hoover's to fill in Account info, if that's what you really want.
Like most of these databases, the contact info is 70% accurate at best. I had a 10% match rate or worse with Data.com when matching against my existing Marketo database.