I have used Windows Live Writer for three posts now, and I’m liking it. Aesthetically, it is very appealing. Functionality-wise, it’s very user friendly and serves well for most blog posts. The WYSIWYG editor is easy to use, responsive, and churns out (mostly) clean, valid XHTML. You have the basic formatting controls (bold, italics, underline, strike-through), image upload (with effects) support, tables, lists, links, and teaser cut off. WLM also handles hierarchal categories and WordPress’ pages. Lastly, there’s the trusty spell check with the squiggly red lines and plugin support.
Windows Live Writer has its limitations though. While it does the formatting functions well, Windows Live Writer has no support for using the other twenty or so XHTML tags like acronym and code without having to resort to the code editor. Even then, WLW tends to destroy code, especially code in pre tags, by removing the new lines and tabs once you do put it in the code editor.
My conclusion: try it if you need a blog editor, and see if it fits your needs, but don’t forget that there are already several others out on the market already.