First off, thanks to Cool Mom Picks for this great review of Grammaropolis. There are so many wonderful lines in there, but my favorite is, "...it's like Grammar Rock and the Mr. Men books had an adorable love child." The comparison is both flattering and, I think, appropriate.
Also in the review, it's mentioned that the ideal age for this is 7+, and while I agree--I've always thought that the target age is between 7-12 years old, right about the time kids are learning grammar in elementary and middle school--this got me thinking about my three year-old and his current fascination with being the Adverb. A couple days ago, he stood up and pointed across the room and told me, "Go over there. I'm the Adverb, so I get to tell you where to go and how to act." In the Adverb's defense, I suppose, my son also refers to that part of speech as "the one who shares his sandwich."
You might say that the only reason he talks about being the Adverb is that he happens to live in a house with Grammaropolis stuff all over the place, but I tend to think that the adverbial qualities of his existed before the advent of these particular characters. (And I'm sure his mother, and anyone who has ever met him, would agree. His will is strong; we'll leave it at that.) All Grammaropolis did was give him the terms with which to express himself. He was going to boss me around no matter what, but now when he does so, at least we all know he's being grammatically correct.