Nobody likes Captchas, but getting hundreds of spam comments is no fun. The currently available captcha plugins for Mango would keep most automated comment spam at bay. Every now and then you get a real person filling out the form. At that point you would need another line of defense, such as Akismet. I have used both approaches combined with good results. Using Akismet alone works, but it is not 100% infallible, so I still needed the captcha. Until now.
CFFormProtect is a comprehensive protection system that uses several methods aimed at finding whether or not certain message is spam, without a Captcha! so no user interaction is needed. And Mango has now two plugins that use CFFormProtect to check for spam before a comment is entered or before an email is sent with the FormToEmail plugin.
CFFormProtect by Seb Duggan. It includes a nice admin page to tweak its settings.
CFFormProtect by Anthony Fojas. (it requires you to download CFFormProtect separately)
Note: They have the same name, so they would conflict with each other if you install both.