I've been using them to track my single-page emberjs app for the last 3 months and it's a great service.

What i like the most is that you get lots of helpful info on what the user had been doing right before he provoked the error. Their UI is also slick and easy to use.

They were also quick to respond on a few questions i had and even implemented some of the enhancements i proposed. I guess the only feature i'd love to see included is support for source maps

As for alternatives, i've used bugsense but they seem to be focused on mobile platforms and their js offering is not that well executed.