We're using Disqus on a blog that will launch in a week or two. Main reason I went with it is because we'll have a staff of four of us monitoring all comments (which will be moderated) and their admin panel for approving or weeding out posts beats the poor comments handling native to WordPress. That and it offers features such as IP or email blacklisting and whitelisting, and we can even offer guest commenting so visitors can still use Disqus but need not have to sign up to make a comment.
The only other viable option was IntenseDebate, but I had issues trying to sign up for it (took me three tries).
I will never ever force anyone to sign up to facebook in order to post comments. I despise that company and its operations on multiple levels, and dislike their "walled fortress" way of doing things. It is just a lazy, cheap way out IMHO, and it offers no way to assign a staff to moderate comments. Yeah, some think it's "cool", but they are limiting comments to only one social media network, which is only a subset of users on the entire Internet.