Both of the sites in my signature have it enabled. I'm using a solve media CAPTCHA, but it should work with any CAPTCHA you choose. Here are the features:
Compatible with 4.1.7 + (untested on lower versions but it should work without a problem down to 4.1.4, when CKEditor was introduced)
guest posts are saved in the moderation queue under 'unregistered' until the system automatically removes (admin setting) the post or the user registers
works with quickreply
has settings to change which editor is displayed, including a basic text box (without loading large CKEditor files) to cut down on page load time for guests
e-mail verification compatibility
when gust post is published (before or after e-mail verification)
CAPTCHA on/off for quick reply/advanced reply & post
user is only asked to enter one CAPTCHA - they will not have to enter another one on the register page
header message to users that they have a 'saved post', even if they come back later (cookie based)
If a user posts, creating new posts is disabled until they register - notification of this is in the header
compatibility with spam-o-matic
reporting feature that tracks total posts / replies, abandonment (users create a post but never register), posts marked as spam, and successful posts / replies (register)
So far with the mod being enabled for a month and a half I've seen a 9% increase of new registrations. Results will vary depending on your traffic and subject matter.
Since I paid a lot of money have this modification custom made I will be selling it for $49.99. I will provide support when necessary (upgrades and unforeseen bugs). Please PM me for details.