Their documentation was well organized, easy to search through for the platform of choice and quick work of full product integration.

Bugzilla is free and open source.

Bugsnag groups similar exceptions together displaying the number of occurences so that there's no need to deal with each instance of the error separately.

Bugsnag integrates with a wide variety of 3rd party tools to automatically create issues or tickets. Integrations include GitHub, BitBucket, UserVoice, Trello and many, many others.

Bugsnag can be set up to send email notifications as soon as exceptions occur. It can also integrate with a number of third-party services that can be used for notifications including Campfire, HipChat, Twilio SMS and custom solutions via Webhook.

Clear, concise documentation. Plus docs on cross-domain script errors and enabling CORS.

Drop in place integration with Unity 3D, only a simple API Token configuration on game object and everything was up & running with almost zero code written.

Supports access from various clients such as desktop applications, browser plugins and IDE plugins (for example, Mylyn Bugzilla Connector).