The more difficult it is to register, the fewer registrations you'll get.
On our forum with 1.3 million posts, we don't use captchas, or even email verification upon registration. Just a simple Q&A.
When we uncover organized corporate stealth marketing campaigns (as opposed to random viagra spammers), we find photos of the executives on the board of directors of the parent company behind the campaign, and, with a simple software tool, we allow people to make comics of them, and then email the comics to the executives, in a sort of "reverse" marketing campaign. They love that.
As rollinj mentioned, paid crowdsourcing makes captchas almost irrelevant, and there is plenty of spammer-for-hire work on Mechanical Turk. See: [behind-the-enemy-lines.blogspot.com...]