I have added a box of links on the left for "Monitoring Software". I have experience with Nagios and Zenoss and find them both to be adequate. As with anything IT related you get out whatever you put into it.
If you expect to have a quick easy fix for all your monitoring needs it just won't happen with any package you choose. You have to take time to study it and deploy it correctly. You have to configure all devices the same and keep them up to date. You have to make sure you follow procedures. Above all else you have to document the whole process and refer back to the documentation when you have a question.
Another big problem is making sure to investigate each and every problem created. You have to find out why something was reported and if it was legitimate or not. If not, you have to make sure it doesn't happen again.
False positives will kill months and months of preparation done on any monitoring system. If you can't trust what it tells you, you shouldn't be wasting your time figuring it out.