Well, I must compliment the developer of this plug in. I was earlier using Captcha (maths) & SI Captcha. The bots just walked through them. There was a literal flood of spam. In desperation, I tried this plug-in and it seems to have stopped the bots dead in their tracks.
While I used to get a flood of spam comments earlier, I have none now.
Let's hope the good times last long (at least till the bots figure out how to circumvent this one).
On a side note, I am unable to see any settings for the plug-in under Dashboard >> Settings. I am using the latest version of WP. Is there a bug?