The only thing captcha and similar devices do is make it extremely difficult for the handicap to access the forum. Those who are blind, have cognitive disorders, hearing issues, or visual processing problems, including Colour processing issues are often effectively banned from the site. Red and green blindness issues can't read the captcha.
I have auditory processing issues, but find the letters are so obscured that it is impossible to tell the difference between a o and a. v and u look identical. And, any programer worth their weight in gold, can write a program to read those obscure letters. So, they are 100% useless against the best attackers. Banks already have the software to read customers illegible writing on checks in order to process what the amount was, and type a code on the bottom. I used to do this by hand. Now, a computer does this for me. So, no, these things do not really work very well.
But, with that said, they DO keep out the casual spammer who can't be bothered with being human at all. A persistent spammer, will log in and set up an account and spam away, leaving behind his calling card.
Alternative: The questions are the only thing that makes sense. Make the question to hard, and you still ban those with certain cognitive disorders, but the vast majority can get in. I do not recommend 12 + 18 - 6 type questions or whatever it was. I had to get out a calculator because I was tired and couldn't do it in my head. It was just annoying.
In short, know your audience and who you want to access your board. If you are a gaming site, then you are to expect gamers who are ace gamers, but might have cognitive issues otherwise, possibly. If you are a geek site that deal with physics and calculus, then that question matters less because most people aren't going have the brains for your site anyways.
signed...one who does have a cognitive disorder, colour processing issues (mild), auditory processing issues but STILL managed to graduate from college and set up several websites, including one using HTML 5 and CSS 3, and 3 forums. I am a bit of an overachiever.