Finally getting around to testing out MarsEdit with the new Squarespace account. So far, the setup and config has been automatic and completely hands-off. I just entered the blog URL and it auto-sensed pretty much everything -- which service I'm using, the API, categories, etc.
The MarsEdit UI is so far pretty intuitive and easy. I'm sure there are lots of options to explore (first up - scheduled posts?), but it's nice to just have a list of previous posts and a decent editor for simple text entries.
Positioning images precisely seems like it will take some practice, but you can't beat drag-and-drop-from-Finder for easily adding an image to a post.
No clue whether this (combined with on-going tinkering with the Squarespace control panel) will prompt me to breathe new life into the blog.