I run a Travel Blog host, and get several hundred spam attempts per day, accounting for more than 90% of the posts on the site. Still, I refuse to put CAPTCHAs in between my users and what they want to accomplish. It's just a terrible use experience.

Instead, I use a combination of human detection scripts, bayesian filtering, and moderation. Combined, this keeps the site pretty much 100% spam free from the perspective of our end users, and more importantly, Googlebot.

More details here:

http://www.expatsoftware.com/articles/2010/03/care-and-feedi...