I am a complete convert and exclusively use the Sibley eGuide in the field. Its the perfect reference for birding in that it packs all the information in the large Sibley Guide into your phone. Considering that youre likely taking your phone out in the field with you, this means you carry around a massive field guide with absolutely no weight added. The compare feature is a great learning and teaching tool as well. I frequently use it when I am leading field trips to explain subtle differences to field trip participants. It is easy to use and elegant. The only detraction would be that the Sibley Guide lacks much in the way of explanatory or background text. What text is present is very brief; however, this is a complaint with the Sibley Guide in general rather than the app. Those who use and enjoy the Sibley guides to birds will find little lost between the books and the apps and much to be gained: more frequent taxonomy updates, a library of birdsongs and the capacity to limit your options to a particular state or province.
SETKacir about The Sibley eGuide, v1.8