We are currently evaluting AHM, having gone through a reasonable selection of other monitoring software (nagios, big brother, paessler ipcheck, a bunch of others...) and so far it's the best of the bunch, in that it gives you a lot of flexibility, it's mature, well-supported with a good community, and reasonably priced and licensed. It has lots of esoteric options that make perfect sense to us (little things, such as repeating a sound until an alert is acknowledged, or running two monitors on one site on different ports (yes I'm looking at you, IPCheck)) and yet the competition don't see how they could be useful. The remote agents work simply and reliably and are pretty clever. The entire alert system provides enough choice that the only way you could be unaware of problems monitored is by kicking that nasty oxygen habit. Views are a godsend, although the main interface is colour-coded in such a way that a quick glance conveys a lot more than just "something is broken". The web interface... well ok, so nothing's perfect
The only real complaints I have are the documentation could do with some more examples, and that it took me so long to find this great piece of software! I had been googling for ages and not coming up with a suitable product, it was only though chance that there was an article on datacrash.net that mentioned HM.
All in all, first impressions are that it is an excellent product that lets you set up monitoring remote sites across the internet quickly and easily, which is exactly what we needed.