1.6 is said to be the Web 2.0 version of Wordpress. The editor is totally remade with lots of AJAX and Javascript to make it interactive and responsive. Other features include a built in SPAM filter. The best thing about it is that it is already building a user base and there is already enough competition among wordpressers to motivate them to blog.
I really dont think it is unstable anymore because the number of users are increasing day by day. It had some issues ofcourse when the service was available to a selected few (I was one of them) but now after lots of fixing, it is going to the masses and seems like it is doing a good job.
P.S. My invite is already given to someone.