I don't really know how many stars to give this book. When it was originally published, it would have deserved four or five stars. Now, to be frank, it only deserves one star if you are interested in Tolkien's languages as such. Well, let's make it two stars, shall we?
When this book appeared in the late seventies, it was about as good as it could be. The authors were competent and tried to analyze the entire available corpus. However, TONS of new material about Tolkien's languages would be published in the eighties and the nineties. Why, this book even predates the Silmarillion!
The real revolution in Tolkienian linguistics occurred in 1987, about a decade after _Introduction_ was published. Then Christopher Tolkien published the all-important source document "The Etymologies", his late father's main listing of Elvish vocabulary, in the History of Middle-earth book _The Lost Road_. Almost every analysis of Tolkien's languages predating this publication was rendered instantly obsolete.However good and plausible the theories set out in _Introduction_ were when this book first appeared, almost everything has now been obsoleted. Even in the cases where the theories actually turned out to be correct, a present-day student would want to know that this info is indeed "Tolkien fact" and not post-Tolkien speculation. At least 80 % of what we now know about Tolkien's invented languages was quite unknown when _Introduction_ was written and published. I maintain a Tolkien-linguistic web-site, Ardalambion, attempting to present more up-to-date analyses. But even now, very much of Tolkien's linguistic material remains unpublished, and it will probably be decades before all the sources are available and any "definite" presentation of Tolkien's languages can be attempted. I, for one, would be very hesitant to publish anything on paper in the meantime.
Just about the only part of _Introduction_ that has not been hopelessly outdated is the discussion of the two main writing systems, the Tengwar and the Cirth. Yet the info in this section is merely a rather more readable presentation of the very dense descriptions provided by Tolkien in Appendix E of the _Lord of the Rings_ itself. Even this section of _Introduction_ is no longer a "complete" discussion, since much material about yet another Elvish writing system -- the Sarati of Rúmil -- was published only this year (2002).