Many people have complained about Typekey, how it has lost them commenters. I am currently beta testing out the next MT-Blacklist, so my use of Typekey will be slightly different.
None-the-less, I have Typekey as a method by which commenters can be assured of seeing their comment immediately show up on my weblog. In the new Blacklist, there is something called "Typekey Free Pass." If enabled, comments by TypeKey-authenticated users will not be scanned for blacklist matches, old entry moderation or max URLs. In this way I am putting a lot of faith into the Typekey system. Un-authenticated users will still be able to post, by MT-Blacklist will check the comment for a variety of things before the comment shows up on the site.
Before I had the opportunity to test out Blacklist, Typekey worked in a very similar way. I had un-authenticated comments set to moderate while Typekey authenticated users would be free to comment without moderation. I think most people have a similar setup but many ask why it isn't possible for there to be some mechanism for the weblog author/admin to have to approve an un-auth'd users once only.
My response is usually, how would you do this ? Would you check the person's IP or would you check their details like Name, Email and URL. If you say the former then what about dynamic IPs. If it was the latter, those details could be spoofed by a spammer.
Most people now days don't mind if their comment is set for moderation because many people know about the comment spam problems facing most weblog authors today.
If you don't like the idea of your readers having to signup/signin to a third-party service check out Tiny Orwell 1.0
Tiny Orwell is a simple TypeKey-like authentication system written in the spirit of the Trackback reference implementation and blosxom. This script provides an alternate to TypeKey for small groups using MT 3.0D or any other tool that supports the TypeKey API with a minimal amount of fuss. It is completely standalone and doesn't require MT at all.