From their blog post:
Developers need detailed information and issue reporting to ensure that their apps provide the best user experience at all times. BugSense offers advanced real-time error reporting and quality metrics about an app’s health, as well as correlations between user retention and errors, with just one line of code. Trusted by thousands of businesses, world-class mobile apps, software houses, and independent developers across multiple platforms, it is the leading quality metrics/error tracking SDK
Every developer knows there are going to be issues with their application, and the more feedback they can get, the faster they can address the issue. It used to be you had to encourage your users to get in touch by email and talk you through all the issues, now with more connectivity and background processes automatic systems can gather data with more accuracy and greater speed.
Bugsense is just one solution, but it's a popular one that's proved itself in action for countless developers, and it's great to see it come to Windows Phone this early in the life of the platform.
There are more details on the Bugsense blog, as well as full details on integrating the system into your Windows Phone development process.