Having chased and photographed Lepidoptera for more than 40 years, I am overjoyed that Paul Opler and Jerry Powell put this volume together. For serious amateurs considering a purchase, let me outline your choices:
The previous definitive volume for all of North America, Holland's "The Moth Book" does a marvelous job of providing excellent color plates, drawings, and a great deal of species-specific data including food plants and species ranges in text form. Though it was first published in 1903, amateurs like myself have used the text for more than a century. However, much has changed since this text was published, including new species discoveries, renaming and reclassification of many species, changes in species range, and more. Used copies in softcover can be had inexpensively (~$10), and I even acquired this in digital format along with Holland's other volume "The Butterfly Book" for about $30. It is worth considering if you are on a strict budget and don't need the most current information. This is the only book which covers moths in the Western region well besides the new book.
An ambitious series titled "The Moths of America North of Mexico" was attempted beginning in the 1970s--the intent was to release fascicles to cover different families in detail. That project eventually collapsed, and the completed volumes are very expensive, while many key volumes were never completed. They are full of information, but tend to lack things amateurs desire. For most folks, they are best referenced occasionally at a university library.
This new text by Opler and Powell has all the best features of Holland's book, but better quality photo plates (with sizing guides for tiny species photographed at magnification), and more thorough descriptions. It covers more species. The information is very current. The only thing I miss is Holland's sense of whimsy--"The Moth Book" contains delightful quotes on moths and collectors which make late night studies more fun.
Note that none of these are field guides. If you are looking for such, I suggest you try Covell's "A Field Guide to Moths of Eastern North America" for the Eastern species, which I enjoyed tremendously during a university stint in New Jersey. Western field guides to moths are produced as often as flying purple elephants, but "Insects of the Pacific Northwest" from Timber Press has decent photos if you are interested in that region; it has modest coverage of common medium-to-larger species. Butterfly field guides are easy to find for regions and even some states, but not moth field guides.
So I recommend you purchase this if you want to identify, collect, photograph or study moths in the West and need to look up species information regularly. I recommend this over Holland's text if you work at all with micro moths. If you want a "field guide," the best I can suggest is a used copy of Holland which will be bulky and heavy in paper, or a digital copy can be put on your travel laptop/reader if you carry one. But you will probably still want a copy of this book awaiting your return home.
In simpler terms, if you are reading this you probably know you want the book. So just buy the darned thing!