I use Merlin to identify calls, especially in spring when I have to relearn most of them. Also, my phone picks up sounds that I can't hear. It's fairly accurate and getting more complete, but I wouldn't rely on it to id an unfamiliar bird.
I started birding way before the internet and phone apps, so I don't use them much. I can usually narrow a sighting down to a couple of birds. I have the Sibley guide app on my phone, and it's handy for checking field marks or comparing two species. And the All About Birds website (also Cornell) provides nice comparisons of similar birds and has links to the Macaulay Library which has photos of nearly every bird in the world (a few to hundreds of thousands depending on species).
I did play around with Merlin using photos on my computer screen. I even tried it on some first-year gulls. It always gave me the usual suspects, but the first species listed confirmed my id.