I found this plugin easy to install, quick and easy to configure, comprehensive in its protection, and effective. So far, no successful DDOS attacks like those that brought my web sites down in August, September, and December. Sure, I'm on a shared server and some other guy might use less effective protection and hose the rest of us, but at least I'll know I'M not the guy who left a door open.
Still, I would not recommend this plugin for novice WP users. You can lock yourself out of your database and web site. This plugin also permanently alters WP files which leaves me wondering if a future WP update might fail or have issues because of this plugin.
It is EXTENSIVE. Use with caution.