I ran a test between Zoominfo, DiscoverOrg, and Datanyze. Zoominfo brought back the highest number of Contacts. Additionally, ZI's bounce rate between the three was the lowest: about 75% of the emails went through.
If you are doing an Account-Based approach, you can open each Account in SFDC and manually find the contacts you want in the SFDC integration.
There's also a feature called "Build" that allows you to run a list of Accounts via CSV and have it build specific contacts - so if you have a list of Accounts assigned, then you can get a CSV back with all the Contacts you were looking for. This is also a "Dislike" - see below.
Customer Service is good, not great. The Support Staff is a bit slow to respond - but there's a phone number that you can call to get immediate-ish help (they'll call you back if you leave a message). I found that talking to my Account Manager was the best way to get good help.
Their product is fulfilling probably 80% of their vision - but certainly misses the mark at times.
For example, using the Build Tool would save my reps a ton of time - IN THEORY. But the reality is, there's no substitute for the accuracy you get when building accounts one at a time. For example: when I load a CSV with 100 accounts with the intention to get 3 contacts per account, I'll end up with 200 contacts. Upon further inspection, I'll find that only 60 of the accounts pulled in contacts, and then some of the accounts pulled in like 12 "CEO's". There's ways to restrict what's pulled, but ZI doesn't have a way to guarantee that the ONE CEO it pulled is the one that you need. So then the rep uses up time to double-check the Build Tool results... and then it uses as much time as it would have if they built the Contacts one account at a time.
Another issue with the product is trying to deal with companies that have generic-sounding names. For example, if you pull up an account with a name like "Bridge Group", you might end up pulling in the CEO for "Bridge Consulting", the CFO for "The Bridge Group", and the COO for "Bridge Investments". This is the same with organizations with multiple subsidiaries with similar names... try sorting out "YMCA" or "Goodwill Industries", for example. They have multiple Account names, website URLs, and email domains. It's a mess with ZoomInfo.
The biggest complaint for me is that there hasn't really been much innovation since buying ZI over 3 years ago. They have a new UI (v2.0), which is nice, but a lot of it is still a little buggy.
I end up keeping Zoominfo thought because of the end product, which is accurate Contact information.
Building Contacts for existing named Accounts within Salesforce. Updating Account information and keeping Contacts up to date.
Because of ZI, we have been able to prospect into accounts at scale. We have also been able to get the majority of an Account's Contact information and in theory cut down on the busy work for my team.