I have been using co.mments for few days now, and I love it. I allows you to track all the comments you post on other blogs in one place.
I read allot of blogs, and I comment often. But if a given page does not have some sort of email notification mechanism, I almost never go back to check if someone replied to my comment. This is why I installed this feature in my blog – to let people know that I read their responses, and that I reply to them.
I have yet to find a site that would not work with this little web service. I highly recommend it.