I used Xerneas in my first playthrough of X, but that was mostly because I wanted to play around with a Fairy-type. I liked the design for its distinct Princess Mononoke feel from the moment I saw it, but I wasn't as excited for it as I was for the fourth gen titles.
I'm playing Y right now and I probably will use Yveltal. It has a very nice design - almost like a sports car, haha - although, again, I find it to be quite underwhelming as a mascot when compared to past mascots.
None of the Legendaries of the sixth gen have really stood out to me as being truly exceptional, though. For all of its hype in the anime Zygarde has been massively underwhelming thus far - unless it gets expanded upon in Sun and Moon, anyway - and neither Hoopa nor Diancie rank very highly amongst my favourite "secret" event Legendaries.
Volcanion's typing I find interesting, and I will probably wait until I watch the movie and actually get my hands on one in-game before I pass judgement on it, but again, its design...meh. I just can't bring myself to really care all that much about it.