System Administrators live in a world of fixing broken, intermittent, and/or frequently ambiguous problems. Therefore, when there's a system that works flawlessly without needing repeated maintenance, we take notice.
PRTG is the most reliable and low maintenance software we have in the office, and we love it. We lean on PRTG frequently to help us identify and resolve issues on other systems. PRTG makes it extremely easy to do so, and we are continuously grateful to have a network monitor that never creates additional work for us, and only ever makes our jobs easier =]
Functionality wise: Out of the box, PRTG monitors just about everything. We had this watching critical components of over 50 servers within an hour. For those things that PRTG miraculously does not support out of the box, they have an EXCELLENT API where you can create and integrate your own custom sensors (powershell, bat, or exe) with ease. We've used this to watch storage pool health, virtual disk health, even the number of CDs and DVDs in our disk printing robot.
Not only can you use powershell and such for custom sensors, you can even run these scripts and programs as custom "notifications". You set up a custom notification to run in response to a certain event (remote desktop unresponsive on server X), and when that event occurs, PRTG can run your script (restart server X) automatically. We've used this on our failover node to restart dell sonicwall whenever it feels like not working (which is rather frequent).
In short, not only is PRTG a monitoring tool, you can also use it as a self-repair tool for your networks and systems. Review collected by and hosted on G2.com.