I was a Jigsaw.com client (which was what Data.com used to be called or rather Data.com is SFDC's replacement product for Jigsaw.com) and had mixed experiences.
Data quality was spotty and provided very little way for us to segment or target companies. So as a consequence, we ended up downloading a lot of companies that used a technology platform that we did not service.
Data quality was spotty - they claimed 75% to 80% accuracy, we found that it was less than that, although it improved over the time we were with them.
When our Jigsaw licenses expired in September of last year, they gave us the hard sell to upgrade on to Data.com. Frankly, they did not seem to make their case very well and really pushed the expanded SIC codes which were of little use to us, they wanted to charge us more, and were going to provide less data.
Ultimately our executives opted not to upgrade or renew, although there were a number of reasons.