One of my blogs which is relatively popular was getting slammed with comment spams (like 50 spams per day). Akismet doesn't catch them because the content of the comments are usually harmless and generic without any links. They are after the comment author URL. It became impossible to manage so I had to modify the files in wp-includes to reject all comments that have comment author URLs. Just disabling the form field wouldn't do it because the spammers are not using the form. Now, the problem is completely gone. I haven't had to delete any spams in a week or so.
What would be a nice feature is a check box that says, "Allow comment author URL only to registered users." This way, if someone is eager enough to want a link to his site, he could register before commenting. Other people can leave comments without going through the hassle of registering. I think this is a good compromise.
Thank you for reading,