Depends on the CAPTCHA provider. Some are good and very readable while others I believe requires psychic powers to read. Try to upload a photo in tinypic.com and you'll see that their CAPTCHA provider is among the best I've seen on the web.
I can't stand CAPTCHA. I am convinced sometimes there is no right answer. I think sometimes they just put jibberish on a screen and they are not even real numbers or letters. Somewhere out there, someone is laughing about our attempts to login without success.
I think the captcha is good for all. All the server would be down if there are no captcha because auto-fill bot would open millions of account or loads of requests to the server.
Now a days most of the servers have audio-sound support with captcha which solves the vision problem. Also, most of the captcha has "new captcha" button which can change the captcha to a better readable one.
Not always and the audio leaves a lot to be desired. Some of them I get right the first time and it tells me that I didn't an have to do many. After the third time I give up because then what I was going to be posted isn't important anymore.