I loved Grammar Snobs. That is, I loved June Casagrande's first book; I have never loved the haughty subjects giving the book its name. Mortal Syntax, however, is no Grammar Snobs.
The concept of the two books is the same: short vignettes addressing one issue of grammar at a time is easy-to-understand ways, comparing and contrasting various authorities, and generally making people who correct your grammar look like ignoramuses themselves. This is good. However, unlike in Grammar Snobs, Casagrande indulges in many too many political rants. That is bad. Perhaps I should be more forgiving, as this was written during the George W. Bush presidency, a time when 80% of us needed to vent a bit (and I do largely share Casagrande's political views, it seems). Unfortunately, it makes the book dated and a bit tiresome at points. I will say, though, that about the book really hits its stride in the last 30 pages, which bumped my rating up from a 2 to a 2.5. I say that as though there is some science to my ratings - there is not.
Bottom line: it's an unworthy successor, but if you enjoyed Grammar Snobs, you'll like moments of Mortal Syntax. The grammar knowledge imparted is solid.