The reaction to Blogger support in MarsEdit 1.1.5 has been great! Thanks everybody for trying it out so quickly and returning with your feedback.
The most common reaction so far has been that it works great but you can't upload images. I researched this and discovered that, unfortunately, Google does not currently support this for "external editors." I found this post on the Google Developer's group from Pete Hopkins of the Blogger developer team:http://groups.google.com/group/bloggerDev/msg/d36fc1fc994d0de6?dmode=source
In it he replies to the question with the statement:
"Not now. This is a very requested feature, and may appear at some point."
So the best advice I have for MarsEdit users who want to see this support for Blogger, is to get in touch with Google and "vote with your email."
UPDATE: This feedback form was suggested by a Google employee as a good choice:http://help.blogger.com/?page=troubleshooter.cs&problem=&contact_type=wishlist&Submit=Submit
I think the thing to ask for is "Image uploading support for clients using the Blogger Atom Publishing Protocol."