As I've noted before I'm using disqus for my comments. Disqus is an external comment system, it can be embedded just about anywhere.
There are some nice points and some negatives right now.
As an external service disqus is completely separate from my blog. I don't need any local auth system, I don't need to store stuff in a local database, and I don't need a server-side language. ;) In other words, it works beautifully on sites built out of compiled static html. And you can even include it into random web pages.
The biggest issue right now is more technical. Other than the fact that some post urls and thread messages are screwed up because disqus doesn't have a way for me to update the bad values initially given the universal js code is atrocious.
The universal js code disqus uses makes use of
document.write so as a result
you can only include the scripts in the place where the output will go, and the
scripts are dynamically generated so we get no benefit of making the css and js
that are common within the scripts external and cached efficiently
(oh ya, right, that document.write usually includes an embedded style tag that
gets outputted next to the markup (btw, this is invalid in XHTML served with the
proper mimetype, so people who are still trying to cling to the old XHTML hype
might not be so well supported (then again, no-one really uses XHTML properly
anyways, so I guess that won't matter to them anyways))).
The other issue there is with discus' API documentation as well. It's not that great... They explain the different api types, and the keys, but they don't give enough explanation. It took me awhile to understand that thread_id is a numeric thread_id I need to query for in the api, not the thread identifier which I defined for threads. Getting forum posts notes two parameters that take entry types to exclude or filter to but they don't list the possible values, the example only lets you know of two possible values. And finally the return data is not explained. Complex data such as what data is contained in a comment entry and the many possible variations that should be formatted differently has little to no documentation at all.
So in summary, discus is a great and has great potential, but some of it's technical stuff like the Universal JS, API, Documentation, and various features which are missing (like multiple e-mails, claiming anon comments, better comment importing, updating bad thread urls and messages, and so on...) need to mature.