I'd suggest the Mollom module instead of the CAPTCHA one. You have to sign up for their service, but there is a free option with a 100 comment per day limit (rejected spam isn't counted towards the limit). The difference with Mollom is that it only displays a CAPTCHA if their service determines that the comment might be spam. It's outright rejected if it's obviously spam. This means that for any normal user posting a comment, they don't see any difference (no CAPTCHA etc).