This service is wonderful. I have only just started using it, but am very impressed. coComment is exactly what Web 2.0 is about in that it increases the ability for users to hold coversations between websites. It allows you to track all of your comments from various websites (even those without comment features). It also allows tracking of whole conversations. Additionally, it seems to integrate very well with most major browsers and platforms.
Once I accumulate more conversations, I am probably going to replace the recent comment feed on my sidebar with my coComment feed.