I've been evaluating the MarsEdit OS X blog writing tool since November 2004. In 2007 I gave it a good try, but it fell short. Finally, in May 2010 I bought it, but by July 2010 I threw it out. It didn't measure up to my lost love - Windows Live Writer .
Six years is a long time to evaluate a product, but not as long as I've been fighting with Google's incompetence. Evidently billions of dollars aren't enough to create a rich text editor that knows the difference between <div></br> and <p></p>. This past October Blogger broke me. While I researched alternative hosting solutions, I decided to give MarsEdit another try. By this time it was at version 3.1.2.
MarsEdit is now good enough. Try it, buy it ($40).
No, it's still not the equal of Microsoft's free Windows Live Writer, but WLW was one of the finest pieces of consumer software created in the past five years on any platform. WLW is one hell of a high standard. MarsEdit is now about 60% as good as WLW, and that's more than good enough.
The key to succeeding with MarsEdit is not to mix MarsEdit posts with any Blogger product. Don't use Google's BlogThis! to create a draft post, use the "Post with MarsEdit"/Blog This bookmarklet that comes with MarsEdit. Don't touch your posts with Bloggers pustulent editor, open them with MarsEdit . When you paste text, always use "Paste and Match Style" (sure wish that didn't strip out URLs though).
You can alternate editing a post with MarsEdit and WLW by the way. The post formatting will not be injured.
There are several bugs with the rich text editor portion of WLW. I don't see them very often because I've learned workarounds. Sometimes you just have to switch to the HTML editor to remove <div> tags that seem to confuse MarsEdit -- though if you stick with WLW (Windows) and MarsEdit (OS X) you won't run into this problem. There are bugs with <blockquote>, it helps to include an extra line before the paragraph.
The good news is that you can learn to work around the bugs, and there's an end to them. Honestly, I rarely notice them any more. I notice more the rich text editor's lack of keyboard shortcuts or menu bar icons for commonly used commands. Daniel Jalkut is actively working on MarsEdit though, and I think they'll be there in the next release.
MarsEdit has made my life better. It's not perfect, but it improves. Within a year or two I bet it's 75% of WLW, and that's way more than good enough.
 Lost because I'm pure OS X at home, and because Microsoft has abandoned the product. Since it was about perfect though, it will be a fine tool for years to come. They've relabeled a "2011" version, but really there have been no changes.
 Ok, so I do open quite old posts with Blogger's editor.