I honestly think that Decidueye is good but NOT THAT GOOD as many people is thinking right now. I guess people want to believe is good just because is a clearly fan-favorite. It has a good niche but I don't see Decidueye being in the OU tier but it'll probably shine in the UU tier just like other starters. I think that people always want the starters to be good but they never have been really good as many other pokémon. The only viable starters are the ones with a mega evolution or with an excellent hidden ability (for example M-Charizard X/Y, Blaziken, Greninja, M-Venasaur and Serperior) but without mega evolution or those hidden abilities they would be outclassed. I'm not criticizing Decidueye, it is my favorite starter in this gen, but let's be honest: all the roles it can assume, other pokémon can do it better. The only thing that makes Decidueye desirable is its signature move spirit shackle, nothing else. It would have been an excellent mon if its SpA stat were in the place where its Speed is but it would have been "too overpowered" or very emphasized on only a few sets.