I love the ability to get notified of an exception automatically before a user has to report the issue. Because it enables me to also know which user experienced the error and essentially everything I need to diagnose the problem, I'm often able to follow up with a user about a problem before they report it, sometimes notifying them I've fixed the issue. This is invaluable because users don't always report problems.

For side projects their free plan is perfect because it enables me to keep an eye on things with no cost.