It's definitely a book worth having, but... I was a little disppointed with it. I thought the index was lacking, it didn't read terribly well and it didn't go into topics with as much depth as I had hoped for.
A book this large should have an index a mile long. It doesn't. Famous people, chemicals/chemical reactions, ingredients, really just about any unique word should make it into the index. A few times I returned to the book to look up a topic I had previously read about and when I went to the index, I couldn't find it easily. For a reference book of this stature, that's unforgiveable.
As far as it not reading well... I remember being completely transfixed with the first version. I believe I read it from cover to cover in one sitting. This one took a lot more effort to get through. Sure, there's a lot more information and the information gets pretty technical, but overall, it lacked a certain liveliness of the first. The first book had an almost playful, revolutionary quality to it. A trickster element. This book just isn't as fun. Gobs of information, yes, but less fun. It's probably not going to happen, but he should write a book with Alton Brown. Alton Brown, although lacking in Harold's sheer informational bulk, is always good for an entertaining read.
Lastly, considering the size of it, this may sound strange, but I found it a little short. A lot of the topics leaned toward an introductory slant. When I briefly corresponded with Mr. McGee a while back he mentioned whittling down 2,000 pages of research to get to the 800 pages in this book. That's the version I'd like to see.
Now, it's very important to add that I just about worship Harold McGee. I feel that he surpasses even Escoffier as being the person having the greatest impact on food knowledge in the modern age. Needless to say, my expectations were gigantic. Don't get me wrong, I really liked the book, but as far as being the food science reference bible I was hoping for... no.