When I use si Captcha I was getting several dozens of registered user per day.
I used another plugin (s2member) that requires (if you use there pro forms and use their s2member pro) to use reCaptcha.
My registrations have dropped to two per day. Even I get locked out because of too many failed attempt to login to my own site.
I wish s2member would switch to si Captcha on the preforms.. anyone that would say otherwise may be loosing a ton of registrants and money.