First, I think the price is a bit on the extravagant side for an app. 
Having said that, since it is a family sharable app I will be buying it.

It will work on anything from an iPhone 4S up, but it works better on the 5S 
and the 6'es.  The 6S even though it has physical stabilization does not 
give any real advantage over the 6 as far as how the KNFB Reader performs.

Now, I have had good luck using my iPhone with other print recognition 
software along with a scanstand.  Lighting is quite important for 
recognition, but with a scanstand, it is possible to get comparable results 
to a flatbed scanner; this is on software that is quite inexpensive.

Now, paying twenty times more for the KNFB Reader will allow me to scan with 
more forgiveness as far as lighting and allow me to get the field-of-view 
reports and to insure I have the phone level.  That is the biggest factor I 
think that blind people experience when using a hand scanner is making sure 
they don't have the document skewed; good spatial perception is critical to 
achieving qualitative results.

If you have an iPhone, use the split-tap method on the left of the screen to 
take a picture and on the right side of the screen for the field-of-view 
report.  Split tapping isn't as likely to jostle the phone as double-tapping 
would.

Steve, K8SP
Lansing, Pure Michigan