This is a copy of a review I wrote for the eastern edition, but I think it fits them both.
I really wish I could agree with the glowing reviews given by others, but I can't, at least in some respects. I grew up with the original Peterson field guide, and it was my parents' bible. The revisions over the years greatly enhanced the original material. I doubt there is a "seasoned" birder out there who would not say that the Peterson guides are responsible to a great extent for their love of birding.
As soon as They were available I signed up for the pre-order of both the eastern and western editions. I have had them now for around 5 months, and they have never left the house. I can only really comment on the eastern edition, because I never had a previous western ed., but I assume this applies to both.
The book's content is at least 95% the same as the previous edition. I have spotted an added picture or two, but not many. Colors have been changed slightly, but I am not sure that they are better, and it may just be the printing process. The text is updated to agree with current information, bird names, etc., but I haven't noticed much else. In that the pictures and information in the guides has always been excellent, all well and good.
The complaint I have is that the book is just no longer a FIELD GUIDE to me, as past editions were. It is thicker, somewhat heavier, and for a very poor reason, in my estimation. The difference is primarily in the back section of range maps, which has almost doubled in size. It takes up roughly 1/4 of the total size of the book. Now, we all refer to a range map from time to time, but I would bet its something like 1 in 300 times we use the book. Beyond that, the regular pages have smaller maps for the birds which suffice very well at least 95% of the time. To waste all that space and weight is ridiculous. If it is necessary to include all those large maps, I suggest they should be published separately and packaged with the guides, letting the user decide whether or not to carry them. I'm betting not 1/10 of 1% would. The old guide slid nicely into a pocket of my field pants. I won't be doing that with the new one, I'd be afraid it would rip the pocket out, if I could get it in at all. The newest National Geo. guides are top notch, and they are smaller, thinner, and lighter than Peterson, as are others. The "big Sibley" has become the bible for most birders, although mine will never leave the house or car because of size, so that leaves out the new Peterson from any primary use other than possibly the "bird feeder birder". My feeling is, the people at Peterson "just don't get it" as far as their niche in the guide book business goes. I feel guilty for being a detractor of this "new standard", but I would feel more guilty if I did not.