reCAPTCHA is long since too much a liability for my liking, if humans typically draw a 50% success rate (vs 25-30% success rate for bots), and the fact that it allows for off-by-one errors... no thanks.
We have a custom CAPTCHA that I wrote, draws 9 or so visibly distinct styles, including some animated ones. Sure, they're breakable, but not trivially or easily. It's been in use on arantor.org now since August and while I've had spammers sign up, I've had the grand total of 3 spam posts from a human spammer.
The other thing is that not everyone has a Gravatar, so while blog users (especially WP users/converts) will be fine, it's not a safe assumption for the purposes of combatting bots.
There are alternative methods, some of which have been integrated already ;)