OK, so under different circumstances, this may have been enjoyable. But I have to rate it as it appealed to me, and from that point of view it was an irritating waste of time.
We travelled there in a camper van and parked nearby. It was the middle of August, but it rained really hard all night - so we didn't sleep very well. The next morning it was still raining - only drizzle, so we thought we'd give it a go. On and off the rain got worse, and then better, then the wind picked up. The up hill climbs became very irritating and given the weather conditions we couldn't really take any good photos. I must admit, the views were very nice and the rocks very impressive - on a sunny day, probably very enjoyable. After mentioning our experiences to friends it seems everyone knows that it always rains in Brittany!! I didn't!
So in conclusion, come when the weather is good, but even then it's a lot of effort for not an awful lot of enjoyment.
https://www.tripadvisor.co.uk/ShowUserReviews-g207340-d1740264-r306519087-Sentier_des_douaniers-Perros_Guirec_Cotes_d_Armor_Brittany.html#