kbhomes's TextCaptchaBreaker shows why text CAPTCHAs are basically bananas. They are often too easy to crack and especially automated - and this code shows a very nice implementation of it. My old blog still has text CAPTCHAs and gets along quite well with them, but that's probably due to the low traffic - in the last few weeks, more and more spam comments have been posted there and I'm not sure if they weren't placed by bots.