Trecile's questions:
Is this purely a mapping app? What I liked about Buen Camino was the ability to plan my stages, the information about towns along the way, lists of accommodations in each locality with the ability to call/email them from within the app, the bar at the top of the page that showed me how far I was to the next town in real time as I walked, etc.
My responses:
OsmAnd can be used to plan stages and indeed route planning and navigation for all sorts of users is its main purpose. By turning on the radius circles you can get a rough idea of the distance to villages and towns ahead. If you then pick one of those towns and tell OsmAnd to navigate from your location to there (e.g. by foot, for pilgrims) it will plot a route, show the route on the map, and calculate and display a much more exact distance to the destination. Another screen will show the elevations and gradients.
As I travel the route, once per second OsmAnd recalculates the route and the distance and estimated time remaining to the destination, and shows both on the screen. Deviating from OsmAnd's planned route is not a problem, the app simply recalculates a new route on the fly. It can be useful to turn on voice navigation so that the system can tell you when you stray too far off the planned route.
By turning on Points of Interest (POIs) for categories of interest at the moment (lodgings, cafes, cultural sights, grocery stores, museums, etc.), their locations become prominent on the map. Enabling the Wikipedia feature allows one open the relevant Wikipedia entry (if one exists) for such POIs directly from the map. I suspect that one must be on-line to open the Wikipedia links; I just discovered the Wikipedia feature recently so not yet an expert on that, but it seems to work.
When reserving lodgings I use OsmAnd and sometimes Google Maps to make a short list of candidate lodging places and then use Booking.com to make the reservations. No one source can reliably show all lodging places; it is good to use multiple sources. One must be on-line to use Booking.com of course. Since I prefer to book lodgings a few days in advance, I use the WIFI from whatever lodging place I am in when making the reservations, so that is not a problem for me. I download the Booking.com reservation confirmation forms immediately after making the bookings; they include the telephone numbers and fairly-exact locations and GPS coordinates of the places where I will be staying.
BTW I use an 8" Android tablet with WIFI and its built-in GPS receiver for all of this. I don't have a smart phone but do have a small iPod Touch and basic cheap mobile phone as emergency backup digital devices. And I ended up never using any of the costly and heavy printed paper Camino route guides and maps that I bought when planning my first Camino.
I have been using OsmAnd+ intensively for more than three years and keep discovering useful new features. I find it extremely useful. Its utility is not limited to pilgrimages, or even just hiking, it is a general-purpose navigation and map app for all types of users: walkers, cyclists, drivers, aircraft pilots, boaters, etc. The developers are reasonably responsive to improvement requests. I paid full price for the "+" variant of the app (the basic version is free) but the price was economical anyway. Occasionally it is on sale for 50% of the regular price.
Whatever you decide to do, have a great trip!