It depends on how you look at them, I guess.
Wally is one of my favorite characters in any of the games. And to me, the fact that he could be called a "rival" at all is sort of cool, really. He starts out as just this sad little kid who you help. And he starts out with a Ralts, of all things. Especially the first time I played the game and hadn't learned how great Gardevoir can be, I just felt sorry for him - poor little bugger couldn't even manage to catch a powerful pokemon. But then when he pops up again on Victory Road, and really IS a rival - really does fight hard and really is surprisingly hard to beat - I think that's just a really nice touch. I'm proud of him for coming that far.
May/Brendan - I like May. She's a "friendly rival," and that's a nice change, especially coming to these games, as I did, from FRLG - where Gary pops up at the worst possible times with his overleveled team and is just a jerk, from beginning to end. May's sort of sweet and charming and I never feel like battling her is a "challenge" in the first place - I don't feel any burning desire to just beat her into the ground like I do with Gary, just because she's nice. So the fact that she's not that tough a battler is okay. And I'm always happy to see her, just because I like her. Brendan's the only one who sort of disappoints me, I guess just because I expect him to be more of a rival. Probably just a male chauvinist thing.....