I'm just back from traveling in Central America and Mexico. On the way down, I had my backpack in the cabin with me as we were leaving immediately from our arrival city. However, because of some liquid purchases (wonderful Guatemalan honey) that could not be brought into the cabin, I checked my pack for my return trip. Well, as I'm well aware of the possibility of this happening, my pack didn't arrive on my flight with me. By the time it was all said and done last night, it took about two hours to fill out the delayed bag form and leave the airport without a bag. My pack also did not arrive this morning on the next flight! Maybe tonight? At best, by the time it will get delivered to me, it will be on the third day or later after arriving home. Because one can travel light when walking the Camino, choose a backpack size that allows you to bring it into the cabin with you. My 36L Osprey has never been questioned as too large for carry-on. Someone commented recently on the Forum that they brought their 47L pack into the cabin and that the overhead bin accommodated it nicely. What you don't want to happen is to arrive in France or Spain and not see you pack on the luggage conveyor belt. If you plan on immediately leaving Madrid or Paris, not having your gear is a real problem. Starting your Camino without your pack or waiting days for it to catch up with you are problems you don't want to deal with. So, my best advice: don't check your backpack! Buen Camino!