3rd Gen May and Brendan

I loved both Brendan and May. Even though they are technically "rivals", I love how they get along. Plus, I loved May's design so much that I tried to dress like her when I was younger. xD My only gripe about them is that we didn't get to see their starter fully evolved.

As characters I've grown up with I love them both, they're very enjoyable to play as and interact with. However as rivals they really aren't very good at all. They are weak, have little to no plot involvement and very little development either. But then, Wally is supposed to be you 'Rival', not your neighbour. But he was equally as bad if not worse.

Well up to the point of ORAS, but before ORAS.
Both of them are ok and still not way challenging than in ORAS would be since their starter stuck at middle stage instead of fully (ORAS fix this by adding mega stone) :3

Both Brendan and May were okay as supporting characters, but I didn't like how they didn't fully evolve their starters in their final battle with you.

I always liked the two characters as your friends in the game. I personally looked at them more as your casual pokemon researching friend, rather than your rival. With May and Brenden as the two characters that you meet multiple times in the game as your friend who occasionally battles you, they always talk about how they are helping out their father researching pokemon and filling up the pokedex, and they always made it pretty apparent that that was their main focus. That's why I don't really mind that they are the least intimidating and competitive rivals throughout the entirety of the franchise, their characters never seemed to be that obsessed with being the best trainers in the game, they just sort of are drifting across the region in hopes to see all of the regions pokemon for the purpose of continuing their fathers research.
It's also, in my opinion, refreshing that there are characters within the franchise who presents themselves as your friends rather than some jerk who says mean things and then tries desperately to beat you at all costs, Brendan and May feel like nice friendly kids, and I actually really like that.
Wally always felt more like the true rival in the game, since every time you meet him he explains that he wishes to overcome his illness and weakness by proving he's the best, specifically by defeating you, which sadly the poor sick boy never does.
I also always liked the fact that May and Brendan always seemed to have a slight attraction to your character, just giving even a slight hint of romance between the characters in the game, even if only implied, makes the characters appear more human and interesting as well as it brings your character to life. The fact that an NPC would suggest attraction to your character makes your character have appeal and charm which makes your character appear more lifelike and full of charisma, rather than how most of the characters were presented in the previous games, where no such appeal was ever really expanded on before May and Brendan.

I really like them as friends, the help you and are always nice. But I just started emerald and they aren't very good rivals. They should have been like Trevor in X and Y. He asks you for Pokedex battles because that's his speciality. And that is wahat Brendan and May also do. But they are better in ORAS. Not very strong, but better. I prefer a good rival like Hugh. I do like their disigns though. Especially May in ORAS.

I actually really like them mostly because they aren't really your rivals. One thing I love about Ruby/Sapphire is that you're on your own pretty much the entire game, and there are very few mandatory interruptions. The fact that they aren't a big part of the game is something I love about it.

May and Brendan were some of my least favorite player characters/rivals. I wasn't fond of the designs, especially the whole bandana/floppy hat thing. I guess I'm just more of a baseball cap trainer. I played as Brendan, and while May was nice, I was disappointed that she stopped challenging me to battles and things like that partway through the game. It was interesting to have "nice" rivals for the first time, but I feel like GameFreak could have done more to flesh out their characters a bit. I preferred the character development of the GSC rival, or the pure rival element of the RBY rival.

Since Emerald was the first game I played, I didn't actually realise that the games were supposed to have a rival, since they were always so nice to me. LeafGreen totally surprised me with that awful attitude.

I believe they are in love to each other upon playing the game. Lol The remakes even brought the ship to a higher level.

On the design aspect, they are very unique and refreshing; the whole attire looks amazing specially for May at Emerald.

I liked both Brendan and May as main characters. As a male, i generally chose Brendan. That said, I disliked them as rivals. Too much frenemy for me. I also was annoyed by Wally. So I didn't really like any of the Hoenn rivals.