My opinion based on different things where a mixed bag. Design wise, they are meh. I think that as a whole they were bad rivals, but right now, during my play-trough of Sapphire to get hyped for ORAS (because i didn't really like gen 3 when it came out for certain reasons) I realized that May, (maybe Brendan too, i just play as a guy) is a ruthless ♥♥♥♥♥. She constantly gets owned, but makes comments suggesting that your journey revolves around beating her, or at the Lillycove battle for example, she pretty much brings up the first battle with her and how i learned from it like she was beating me and i had to learn from my mistakes! I just never saw it before but she is just evil in a nice girl package. Also, lets compare say, gen 2 rival Silver to May/Brendan. Silver is full of angst and has a dream he wants to accomplish, but goes the wrong way about it. I felt like although he started out thinking Pokemon where tools, he finally learned near the end that the only way to get better is to show them love, but was so full of anger that he didn't care. He's a typical A-hole but at least you can kinda figure he has a huge chip on his shoulder to be so angry every time you see him.