There are two things in particular I like about this plugin, and I wanted to sing its praises in case anybody's on the fence about whether or not to get it.
1) There's no configuration needed...you can use it right out of the box. Install it and you get a new widget. Simple!
But for me, the best part is
2) The plugin uses ReCaptcha, which is a Google captcha service. The neat thing about this is that google's captcha service is doing more than just providing words for your users to authenticate against...it's also helping to digitize books, a couple words at a time. How sweet is that? You get protection against bots and spam routines, and help digitize books...that's wins all around, and it makes this one of my favorite plugins!