As I mentioned a few days ago, I ported my custom appengine implementation of comments to Disqus comments. I could have continued to improve my comment system (better spam detection, threading, allowing commenters to get updates via email, ...) but I still would be missing the ability to learn more about my commenters who aren't bloggers as Disqus lets you click through to other comments by a given commenter.
I'm finding myself more active in my own blog and others blogs thanks to Disqus. When I had a few issues importing with data integrity failures from my imports they were fast to fix the issues. Their REST api made batch posting of comments easy. I started with Disqus Sinatra Importer, to which I ripped the Sinatra code out and added code to grab a JSON feed of comments from appengine.
My only wish is that there was a way to grab a static html version of my comments periodically (perhaps only those that I know have new comments via webhooks?), which I could insert into the <noscript> section of my articles. This way users without JavaScript and Google could see the comments without having to click through to overstimulate.disqus.com. Luckily their API exposes everything I would need to implement this myself, so if it becomes a real issue I won't have to wait for them.
