What does BuzzSumo do well? Zeroing in on a topic, influencer, or website's social reach is BuzzSumo's forte. I like the content idea generation tools very much. The ability to quickly track is a benefit.
What does BuzzSumo not do well? BuzzSumo is a premium service to purchase, even at its lower levels. They're not only more expensive than their competitors but unbundle a number of features. I'm not sure whether they do that to create pricing tiers or they have data costs that I don't understand, but they don't do a good job explaining the reason for the premium price.
What do you wish you had known about BuzzSumo before using? The number of seats (user accounts) and projects is pretty robust even at the lower levels. The monthly cost can be split among several small clients if your agency work requires expensing to projects. There are things that BuzzSumo does like backlink analysis that are done much better by companies specializing in SEO so make sure you're buying a feature set that you need.
Have you used a competitor to BuzzSumo? If so, how does it compare? BuzzSumo excels at providing a nice interface for the social Internet including trending topics, influencers, and content generation ideas. They may be the best overall at that group of services. As search crawlers, they're not as impressive.