I have a Dakota 10 and Dakota 20. I mount it on my mountain bike handlebars and it works wonderfully. It runs for several days on two AA batteries and is waterprooof.
I've used mine from Italy to Arles and Arles to Toulouse, and it was essential at times, especially on the new Voie Aurelienne from Italy to Arles, where the marking is not great. We like to stay in Chambre d'Hotes, and we would often follow it to the tourist office, (they are all on the Garmin map) and get info on the best places from there. We'd find motels, restaurants, phone numbers as well as following the route.
I don't think it would that useful on the Frances, but it does pack a lot of info into a small package, and makes a handy record of each day's travel. I can go back and find restaurants we ate in just by looking at the track from that day. I seldom use maps anymore. If the track is good enough, you can zoom up to a scale where you know if you are 10 meters off route.
There are many good tracks on the internet. I put them on Google earth, make a few variations for mountain bikes, then load them to the GPS. I don't think buying a Camino specific pre-loaded package is neccessary, but if it is a good package at a good price, why not? I'm not sure what is in that model, but it might save you the time of loading some of the info, and it might have more albergues than the regular Garmin map.