I recently downloaded the Audubon Birds app to my Android phone. I've been using this app and the Sibley eGuide interchangeably in the field. I've had some great compare and contrast between the two. I really like the Audubon Birds app for a variety of reasons.
User-friendliness - I really enjoy Audubon's thumb-sized circles and icons to tap for accessing the various browse options; especially because of the fat-thumbing issues I have discussed before.
Intuitiveness - this app is just so easy to navigate and it doesn't take any complicated effort to figure it out.
Speed - while it does take a little while to download initially and to load for daily use (it is a large app afterall), once the app is open it navigates quickly and smoothly.
Reliability - I have never had the Audubon Bird app shut down on me or "force close". Unfortunately, that is a big problem for the Sibley eGuide.
Search-ability - Audubon has the ability to search for birds by shape, by name, by family, and by an advanced search tool . Within the "Advanced Search" you can look up birds by their scientific, common, and even alternate names. You can narrow your search by habitat, region, color, size, shape, wing shape, type of call or song, and even by flight or running pattern. It can also show you species by the likelihood in your state or province, even by month! When it comes down to it, I pretty much only use the "Browse by Name" tool, but its cool that those other search resources are available, especially to new birders.
Beautiful - everything from the photos to the color schemes on this app are just really pleasant to look at.
There are at least a couple photos for each species and they are great quality images. Males and females are almost always shown, but it may be lacking in some more birds in juvenile plumages. The biggest weakness of the photos in this app is the lack of text describing what it is you are looking at.
For $9.99, this is a solid birding field guide app and a fantastic value! I highly recommend this app to all birders. It is also available for iOS.