(I'm cheating a bit by saying I "read" this. Actually, I listened to the 18.2 hours of audio lectures that accompany this book in a set. This is a product of The Teaching Company.)
McWhorter is brilliant, just brilliant. A fantastic communicator. I thoroughly enjoyed the entire series, and it didn't hurt that he had me frequently chuckling at his cleverness.
As to substance, McWhorter knows what he's talking about. The changes languages go through—and especially the narrower topic of creoles—are his area of expertise, but that certainly does not exhaust his knowledge. Topics he addressed include "Language Families," "Dialects: Where Do You Draw the Line?," "Language Death," "Artificial Languages," even a topic he may be uniquely situated to address, "What Is Black English?"
McWhorter provided memorable illustrations, like his cat nosing into his empty suitcase as an analogy for how language develops unnecessary complications over time. He took time out to weigh the good and the (mostly) bad in the Sapir-Whorf hypothesis. He made fruitful application of the continuum idea, showing that the line between dialect and creole is not hard and fast.
I would consider this a pretty ideal intro to linguistics for laypeople, and it will contain a lot of food for thought for those who, like me, have already done a fair amount of reading in the area but who aren't professionals.
For biblical scholars, it seems to me that James Barr (and Moisés Silva and Stanley Porter) are still right to say that we ignore linguistics to our peril. We're supposed to be people of the Word, so we must necessarily be people who understand words.