To build a strong prospect list and clean your database at the same, Data.com is unbeatable. Their advanced searching cuts down the time required to sift through accounts that don't meet your ideal customer profiles, and allows your resources to be more effective and productive overall. As an added bonus, the smooth integration to Salesforce will appeal to users of one of the larger CRM's, though; some may find the fact that organizations with multiple locations are hard to differentiate when importing them to your instance of salesforce without creating duplicates.
As far as data integrity is concerned, Data.com does a moderately good job of providing a healthy blend of senior and entry level employees, though you may find some of the information outdated. This can be annoying, but leveraging the available information the right way beats having nothing at all and being stonewalled by a gatekeeper.