I don't know -- I didn't really mind the whole 'several-rival' set up. I thought it was a little more realistic because, you know, friends. I wouldn't start a Pokemon adventure without my friends, and they'd be challenging me to different random stuff all the time.
That being said, I thought maybe their personalities were a little lacking. Shauna was cute, but I wish there had been a little more to her. Like, more conversation about why she likes you so much and such. Trevor was "shy and smart" and those were his only two traits. Tierno... gosh, honestly, he was just "I LIKE DANCING" the whole game and didn't have much to him at all.
I do like Callum/Serena in the sense of always trying their best to get the upper hand (without being a jerk), but they really didn't present much challenge. I started with Chespin, but battling Serena was a piece of cake with my Charizard/Talonflame combo on top of the rest of my team. Further, the thing that really irritated me was her constant lament of losing. It made me feel bad for demolishing her, but not really, because she kept whining. I understand that it was probably there to force character development (as later on she decides she doesn't mind losing and wants to use it to grow as a trainer), but it really got old fast. At least she was consistent in supporting you, though.
All in all I like the direction GameFreak was trying to take and I can't wait to see more in the future; I hope they'll keep improving.