What is Captcha? It's that obnoxious little box you see with the hard-to-decipher letters and numbers you have to type to prove you are a real human. Take that fourth character in the captcha box you see here: Is it an el? i? t?
The last time I left a comment on a blog that used Captcha, the letters were so distorted that I needed to do it two or three times before I figured it out. Usually, if what I type fails on the first attempt, I abandon my attempt to leave a comment on that blog and simply move on.
Feedback from your readers is too valuable to let this happen to you.
You should set your commenting system up so that the barriers are as non-existent as possible. You want it to be easy to leave a comment. If you are having trouble with spam comments, don't shift the burden to your readers. That's being lazy. Moderate comments to deal with the nasty ones and use some sort of ant-spam function to deal with the rest.
If you have done a good job with your blog, the people reading it are your fans. They are emotionally invested in your success. Their feedback is priceless. Don't make it hard on them.