I am writing this through Windows Live Writer. There is a lot of talk on blogs about this at the moment.
I must say initially I am quite impressed. Whilst editing this post it is using the colours and styles of my site.
The applications appears to be extendable, using an SDK. I am not sure as to what is involved in writing an extension but it should be fun. More information about extensions can be found at http://dev.live.com/blogs/devlive/archive/2006/08/...
The speed of the application is quick.
The only other thing is that it appears to have the functionality that most other Blogging tools have so it is already well equipped.
I have not noticed any bugs. It even works correctly on this Blogger account that is stored on my server.
I am going to have a play with some of the pluggins to see what they can do.