Hi Kimi -
Josh and I have discussed this point before regarding match rates and more with this solution, and it's in line with an upcoming case study I have from Steve Susina at Crain who got a 10-15% match rate with Data.com and an 80% match rate with ReachForce DataCleanse & Append solution. ReachForce SmartForms is another data enrichment solution that will update your new inbound leads in real-time. The Marketo configuration allows you total control of what data attribute you wish to be master - no overwriting anything unless you want to. In my own Marketo flow, I push all the appended fields to dedicated stand alone fields and then built a Marketo data program that will look for empty address or phone fields etc and then copy in the SmartForms appended data. If there already is an address and phone (perhaps added in by the sales rep through Salesforce), I wouldn't want to overwrite that.
There are other solutions out there as well.
Mary