I like that it's easy for me, as well as my team members who use this software daily because of its ability to search by several different criteria in one database which makes our jobs easier! The only thing about response source are their monthly fees if you have more than 200 contacts or 500 searches per month they charge $100/month + tax (in California).
We've been using RSMC since 2013 so we can say there isn't anything bad with them but just be aware when calculating your needs before purchasing any products from rsmc.com.