I like how it's easy to integrate with other products such as Nagios. It is also pretty good at alerting you when things go wrong. Sometimes there are some minor quirks that can be difficult if you're not familiar with scripting languages (e.g., using shell commands). We have used Monit for our internal servers along with nagios and we've been happy so far.
One thing i dislike about monit is that its hard to track down any errors in your logs because they all get grouped together under one logβ¦