Capterra is by very far the leading platform in "old school" software categories and outside of the US.
If you're in a red ocean type category and addressing SMBs it's probably the one you should focus on. If not, it definitely doesn't have the hype that G2 & GPI have.
I wrote a short post detailing the gartner review sites, how they work, and how to make the most of it. It might be helpful.