Personally I loved him, Emerald was my first pokémon game and I had no idea what I was doing (mostly I was confused how Pikachu and Ash hadn't appeared yet, although that was partially because when I wrote my letter to Santa I thought FrLg were the newest games). So when you're about to leave the second city after being rejected from a gym challenge (I thought it was because I had bad nicknames :P) this guy turns up, initially suggesting you could be an incredible trainer, then putting you down again? That gave me a momentary ego boost and then motivation to prove him wrong.
By the end, in Battle Frontier he's just a cool guy who happened to have created that tournament one (Battle Dome, I think?) which is still my favourite location in all of pokémon - like if they bring that version of Frontier back in the remakes then I'll go out and buy a 3DS and a copy purely for that reason.