Atlassian Apps (also referred to as Add-ons or less often Plugins) are the organic part of the ecosystem.

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  The usefulness of the given Atlassian App: Marketplace reviews

  As with Google Play Store or Apple iTunes, you can get some user insights about the apps both in statistical and written form. The personas of reviewers are displayed, hence the feedback is pretty reliable, even if not the single source of truth.

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  Compatibility with your platform

  Cloud, Server and Data Center solutions are all supported by Atlassian. The approval process contains numerous tests, and we strive to leverage all of our apps power by making them compliant with all platforms. While our flagship product completely covers those, some others have not gone through the approval process yet. Nevertheless, you can still be sure about the frictionless high performance. As you may see in a Bug Watcher Notifications review, one of our customers is using the server version "in a relatively large JIRA Data Center instance (thousands of users, millions of tickets, etc.)" and hasn't run into any issues since 2015.
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  Atlassian Staff Pick mark

  This is a sign of a great product, as this is given by the Atlassian ecosystem team for free, based on usefulness and quality. The release date of the app and the vendor presence on the Marketplace can also give a hint, as this predicts the experience level.
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  Acquisition of the vendor apps by Atlassian

  Once in a while an app hits the bar of both outstanding usefulness and quality that inspires an acquisition, as happened with our app. Read the full Ultimate Permission Manager for Confluence story here. As this happens, in many cases, it is removed from the Atlassian app store and integrated into the core software.
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  The support and documentation.

  Support and app documentation go hand-in-hand and it can be critical that you get an answer when needed. It's important to note that, based on the SLA (Service Level Agreement), you may get an answer within X days in general. Also, in our experience, there are apps that basically don't need any support, as their functionality is so obvious. This is highly beneficial for both parties to save time and effort.
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  Active Installs

  Popularity is also a good indicator, alongside the other factors. Usually the more people using it, the more experience the app vendor can gather to further improve the app. As a side note, some Atlassian apps need a deeper knowledge to unleash their full value, so the number of installs itself cannot determine the usefulness. For example, our free Advanced Children Display for Confluence needs some smaller macro programming knowledge.