Just wanted to offer my 2c: I really like wp-reCAPTCHA - I far prefer both its appearance and functionality to si-CAPTCHA. However, it would be great if there were an *option* to ignore comments for which the CAPTCHA was failed, rather than to save them and mark them as spam.
Basically, the reason I implemented captcha in the first place was because my SPAM queue was filling up so rapidly. Not often, but sometimes, it would catch a good comment - so I would always check it manually. As it got bigger and bigger this became more and more of a nightmare. si-CAPTCHA solved it completely: virtually no more spam came through, because spambots could not submit comments with captcha.
This plugin, on the other hand, *will* store the comments as spam, even if the captcha is failed (or blank) - thus, the net effect is that it behaves essentially the same as if I have akismet installed, and no captcha at all.
..Unless I'm missing something?