Cheren is a nerd. I mean, the way he's saying things sounds like as if he was smart. Now he's a Gym Leader for some reason I don't know. Once I beat the game, there's not much I can do. I try to do competitive Pokémon in the games, but it is time consuming and not helpful. Even then, Black 2 and White 2 didn't increase my interest in Gen 5. Only the competitive stuff made me interested, as I watched videos about that on YouTube. Tons of battles were on the DS itself, and rarely on the Pokémon Showdown! simulator. Pokémon Online is just as good, though. Same stuff, but with a better teambuilder.
X/Y's rivals were lame. Serena was lame. Shauna was lame. Tierno was lame, and Trevor was also lame. All the joy just to be annoying. This is why they say Blue is the best rival.
Platinum is good. Like, really, really good. The music of Gen 4 in general is peaceful. I'd say it's better than Black and White.
People may complain about the slower framerate, but what can you do about it. It didn't bother me. The Elite Four was much more difficult in D/P, though. Platinum's is a bit easier. The Pokémon availability is better in Platinum, so yes, if you ever get to play it, you won't be disappointed.
Also yes, Mario Kart DS is the best Mario Kart game. No regrets.