Well I will start with the first impression. It seems clumsy at best. I know I am being harsh, but it seems to be out of place on my computer. The one thing firefox got right was that they were not trying to impose the browser on you. You are given a very basic shell which you can extend as you wish. If you like themes you download it, but if you don't, nobody imposes it on you.
That is one place where I see it as a negetive. But in anycase these are very early days and its just a developer preview. So things could change.
Besides that I kinda like this blog editor. I am also making a similar extension for firefox, "ezBlogPad" but I have not even started that as yet. So that will be completed once I start it. Other that that it seems just like firefox with a clumsy theme.
"The theme is soothing and blah!", says xyz. I don't care how soothing it is. It is trying to fit in with all my other programs and it looks like an alien.
But that is just my first impression. Will write more once I see more of it. And I won't trash it, 'cause I know there is a lot of effort that has gone into making it. :)