The API is well documented and relatively easy to use. You can use elastic search queries to do finely tuned searches. Also, their upcoming Company Insights endpoint will be a game-changer in data analysis of a companies' health and growth. Review collected by and hosted on G2.com.
There are some data quality issues. For example, the data seems to trust LinkedIn a bit much. I have found cases in which random LinkedIn profiles would say they are the company's CEO, but that doesn't match public sources. Also, if a person is not on LinkedIn, they will not show up in search results or be counted towards a companies' employee total. Also, there is no GUI for querying the data, just a GUI for building the query string. Review collected by and hosted on G2.com.