I have been using WPUmbrella from its early beta releases until now. It is very easy to use, it works as advertised, and it's one of the more robust monitoring solutions available. You can do things like set up alerts for low disk space or memory usage, which are pretty important for a web server. There are some reports that don't work correctly (such as graphs), but those issues seem to be very rare if you're looking at the overall system health. We've had issues in the past where monitoring tools weren't giving us enough information about our applications, so this has been really helpful.