I have 5 sites with simple math captchas and get hundreds of spam a day that don't solve the captcha and which are therefore rejected. None solve the captcha.
What do you mean by "simple math captchas"... where the distorted characters display something like "2 + 3 =" - and I'm supposed to type 5 or five?
Or are you simply using a simple math question (un-captcha'd)?
I've been experimenting with my own db of questions that can be answered with a word - including simple arithmetic and simple reasoning such as "Which is warmer? Hot or cold?" - and finding almost no spammer cares enough about my sites to develop a decoding table for comment submission [that would of course change if I were Amazon, etc.]
All of this is moot, of course, if some spammer were to care enough about auto-registering/emailing my site to get porn-visitors to solve the questions/captchas in order to "see more".
YMMV