I have changed my commenting system to Disqus. The upgrade required me to change the look of my blog. I tried to pick a layout that looked similar to the one I had before.
From personal experience, Disqus can be an aggravating system at times, but it's free, and it's better than the one I was using before, Echo, which is not free. Before I upgraded, I manually saved all the past Echo comments to my computer. I intend to repost them in the Disqus section for old posts, but it will take some time.
I also have a few things that got eliminated from the sidebar when I changed the layout. I intend to put them back soon.
UPDATE (2/24/2011): I have now successfully exported all the old Echo comments to Disqus. Feel free to add your comments to any post, old or new.
UPDATE (3/8/2011): It turns out the comments have not been successfully imported. Only for posts from about a year ago to the present have any of the comments appeared, and even then, it is inconsistent. I'm currently working this out with Disqus through email. But you may still add comments to any post you like, and they won't be affected by the eventual transfer of the old Echo comments.