I have used field guides for many years, and own several from different publishers. However, I find I reach for my field guide apps more often because of several advantages they have over a book.
The ability to search on location and characteristics is one of the biggest advantages. Yes, you can get field guides specific to an area, but they are not usually helpful when trying to identify something that is not a very common species. Birds, insects, and animals do not always stay within their designated field guide ranges. The app gives you the ability to filter on what you know about an organism, which helps narrow down the possible choices for identification.
Another advantage an app has over a book is having several pictures of an organism in cases where it is helpful to see both male and female (birds), top side of wing and underside of wing (butterflies), and variations due to location (birds and insects, generally).
The bird app and the reptiles and amphibians app even have audio samples of bird calls and frog and toad calls. These are very useful because many times you will hear a bird, frog, or toad but never see it.
All that being said, the most useful field guide is one you will use. ?
