I have implemented ReCaptcha and works OK using SS 2.3.3. You need to install both ReCaptcha and SpamProtection modules together. Then run the /dev/build parameters in the URL and you will have the "Spam Protection drop down field for UserForms and if you turn on page comments, it will display the ReCaptcha image as well.
Please note that you have to register for a ReCaptcha key at their site, so that you can get a public and private key assigned to you, which you have to incorporate into your _config.php in the 'mysite' folder. See instructions at: http://doc.silverstripe.org/doku.php?id=modules:recaptcha
You also need to add a line of code inthe _config.php file in the 'mysite' folder. See instructions at: http://doc.silverstripe.org/doku.php?id=modules:spamprotection
This is my code in my _config.php:
// Spam Protection Setup - 'recaptcha' & 'spamprotection' modules
RecaptchaField::$public_api_key = '<publickey>';
RecaptchaField::$private_api_key = '<privatekey>';
Copy and paste it into your _config.php and replace the items <publickey> & <privatekey> with your keys generated from the ReCaptcha site. To register and get your key for ReCaptcha, go to: http://recaptcha.net/api/getkey
I have implemented this using 'olivesunset' & 'blackcandy' themes and they work fine. However, on my 'highground' theme, the ReCaptcha image block is a bit screwed up. i.e. too many line spaces in between the ReCaptcha code. See: http://ev.ulysses.co.nz/guest-book/ I think it has something to do with the line height of the CSS for this theme. I am still to find a solution for it.