I heard about the application from Melanie on the Camino pilgrim forum, and thought for $3.99 it was worth a try!
I purchased the iPhone version and I'm so happy with it that last night I bought the iPad version! I will be using the iPhone version on my walk next month. But the iPad version is easier to read and study now because of the larger size. However, for a total of $8.00, buying both is much less expensive than purchasing a heavy guidebook to pack along.
Melanie has made a wonderful application, and the walking directions appear to be very clear and concise. There are many, many photos of the places you are passing. There are very good maps. And best of all, people using the application on the road can post comments about changes or items of interest.
About the only thing that would make this app better would be a list of albergues in each village. But seeing how that changes from year to year, I think the comment section is probably the best place for that.
So... if you're going to walk the Via de la Plata and you're looking for a guidebook... and if you plan on taking your smart phone anyway.. pick up this application.
Here is a link: VDLP Application