Let me just say that I can't fuckin STAND Yveltal's design. Maybe if the tail wasn't the exact same design as the other two limbs/wings, it'd be better. But as is, it just literally looks like an anthropomorphized letter Y. "Then we put a head on it." UGH!!
Xerneas for president.
I really dig Yveltal, and I wouldn't be surprised if you warmed up to it at some point, as well. (Seems to be how Pokemon designs go for many people, myself included -- see also, Chespin's rising popularity!)
Way I see it, the fact his "wings" and "tail" are all the same is rather unique. I think it's the "hook" to his design, very deliberately done to make him stand out from the rest. And I really wouldn't have seen the "Y" connection if not for his name and the game he appears in -- I'd just see an interesting silhouette, which is key to any good Pokemon design.
The fact his back is different from his underside is nice, too: Black on the back, with red diamond patterns, and red on the underside, with a sort of circulatory system pattern. Five claws on the tips of his wings and tails... Angular horns that recall Terrakion (much like how Xerneas recalls elements of Cobalion and Virizion, ironically)... Pointed snout and the black pattern around his eyes... All good.
Only element I don't quite dig about Yveltal, at this early stage, is his legs just seem to abruptly emerge from his bod... In that CGI shot, at least. But otherwise, I think he looks awesome. I was all about Xerneas at first, but I think I'm now in Camp Yveltal.
I just have to keep pronouncing his name. "Ee - VEL - tall!"