Here is simple criteria for a quick evaluation of apps from

http://www.alsc.ala.org/blog/2011/07/5-questions-to-ask-when-evaluating-apps-and-ebooks/

1. Does it expand and enhance the traditional reading experience?
There’s not much point in offering an ebook or app if it is simply a page-by-page replication of the physical book. A great app or ebook should be interactive and encourage creative thinking and problem solving. It should take the characters, the setting, the themes, or the world of the book and allow the reader to explore them in new ways.
2. Does it allow a linear reading experience?
While cool games and interactive features can distinguish a good app or ebook, it can also be its weakest attribute. As Michel Kripalani, CEO and founder of Oceanhouse Media Inc., a leading children’s app developer, explains, “it is important that the games and additional features have purpose and do not diminish the reading experience.” Kripalani recently related the story of a friend who had a rather disappointing experience while sharing a book-based app with his daughter at bedtime: The app interrupted the story so many times with games and activities that the little girl and her dad were never able to fully enter into the narrative. The magic of that shared reading experience was lost. A well-designed app should strike a balance between opportunities for exploration outside of the narrative as well as opportunities to lose oneself in the story alone.
3. Does it engage multiple literacies and learning styles?
An excellent book-based app or ebook should offer the user a dynamic experience that engages the senses and allows for interaction in a variety of ways. For example, many apps are geared, by their very nature, towards visual learners. What about an app that also engages auditory or kinesthetic learning styles?
4. Is it intelligently designed? Is it intuitive, flexible and customizable?
Just as a well-designed picture book achieves a fine balance between tension and surprise, and expertly employs the “turn of the page” as a means of creating drama, an intelligently designed app or ebook can maximize (and individualize) the unique relationship that exists between screen and user. Above all, a good app will be user friendly and easy for children to navigate. Beyond that, a great app may offer customizable features such as the ability to alter the settings (easy/medium/hard) and accessibility options (such as font size or narration speed.)
5. Does it have legs (i.e., longevity)?
As far as whether or not any particular ebook or app will be technically compatible with future devices and services is hard to say. That depends a lot on the vendor, the device(s) currently used, and how libraries wind up negotiating and reimagining terms of service contracts with publishers, developers, and service providers. Be that as it may, it is still useful to look at the overall shelflife of any particular app or ebook.
We know this to be true when it comes to physical media. You’ve seen those hilarious books that have you LOLing after that first read. After the third or fourth, however….not so much. These “one trick ponies” exist in the digital world as well. A great app has, as executive producer of children’s apps at One Hundred Robots, Matt Bassett, explains, “replayability.” A good app or ebook should entice children to enjoy and explore them again and again- just as a good book invites multiple readings.

Here are some additional things to think about at

http://cybraryman.com/ipadeval.html

http://techpudding.com/2011/04/04/checklist-for-evaluating-technology-software-and-applications/