I've had serious problems with some. The colours of the characters blended so well with the background I just could not make them out. 10 mins and more on a single submission's captcha is taking the piss. At that point I did walk away from three or four of them.
But once I pointed it our in my earlier thread, everybody took a look at their own captchas and those that had bad ones did something about it. So in essence, you're right. It's about getting things in proportion.
And I still prefer the Ms&Ps.
