At Mixpanel, we use http://www.exceptionhub.com/. Much better tracebacks and filtering of useless errors. Tracking of related errors and of browser, referrer, etc. window.onerror is unfortunately lacking in the information it provides. On the other hand, EH is not a free service and it's cool to see creative uses for Mixpanel :D.

Great idea to use Mixpanel events for implementation. Much easier and quicker to implement for the majority of sites.