The Hotel School at Cornell continues to do great things with TrackJS. They’ve written up their thoughts on identifying legitimate errors that’s very interesting.
Non-legitimate ones: These are caused by buggy browsers or their respective plugins. They usually don’t affect site performance at all.
Legitimate ones: The trick is deciding which category your error belongs to. That’s an art, and there are no hard and fast rules. But there are some good heuristics, and TrackJS definitely helps you out there.