The installation and setup of Lithium, while not exactly easy, is not hard either. There are a few steps that can seem tedious, but compared to the installation and initial configuration of things like Nagios and Cacti, Lithium is cake.
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I have some minor nits about the installation. Lithium uses Postgres, which is an excellent database, but I'd like the option to use MySQL, since that is on every copy of Mac OS X by default. Lithium installs at the root level of the boot drive, and that's just annoying if you have a specific way or place you like to install such items. I do however like the fact that Lithium is a self-contained application, so uninstalling is simple. Id like to be able to point that installation folder somewhere else, but it's not, as of yet, a huge inconvenience.
However, if I had to state my single biggest complaint about Lithium, it would be the inability to create custom probes and modules. Lithium is, right now, a gated community. If you use it, you have to live with its rules. This is unlike products such as Nagios and Cacti where you have a nigh-infinite number of ways to customize the way you use the application on your network. Lithium has told me that not only customizable probes, but direct support for Nagios plugins is in the works. Based on the pace I've seen on the application, I have hopes that both of those features will become a reality soon.
However, don't just take my word for it. You can download a free configuration of Lithium and test it for yourself. It doesn't include all the features that the professional and enterprise editions include, but it will give you a solid way to use and evaluate Lithium for your own needs and uses. Lithium's core, which includes the web console, runs on Mac OS X and Linux, and the Console applications run on Mac OS X and Windows.