He's still 100% usable for the rest of the game. He does have that lull where he just does horribly against Driftveil, and he can still be decent against Elesa if you have nothing else.
After my first run:
Magnemite: TOP FUCKING TIER. He means that you can faceroll all but one part of the game, and that's Driftveil Gym. Shows up really early, gets Sonicboom in ONE level meaning it can faceroll the second and third gyms. Will probably evolve just before Chargestone Cave, and can be evolved to Magnezone literally 5 minutes later.
Typing is great, and the evolution is a choice between trading in for speed and getting a ton of SpA, or not evolving and retaining some speed speed. Gotta choose between 120 / 70 SpA / Spe, or 130 / 60. His only drawback is his middling speed and his somewhat "eh" movepool, but that's why you're pairing him up with anti-Ground and anti-Fighting.
Riolu: Mid tier. He performed well, but not well enough. He's a baller in competitive matches, but his movepool is somewhat lacking. (Force Palm? Rock Tomb? Bone Rush? Really?) He's amazing once he evolves as his Steel typing is awesome, but until then he just sucks otherwise. If he got Close Combat earlier he'd be almost top tier just because of that. Still, Swords Dance at lv 37 means he can fix his middling attack stat, so he'd be higher if his movepool and TM availabilty was better. Also, evolving him is awkward. Black players have it better as they can grab a Heracross just before the 4th gym, which is so much better (or is it after? No clue if you need the badge to use Strength outside battle to get there.)
Lilligant: High tier. Terrible coverage, but made up ENTIRELY by Sleep Powder and Butterfly Dance, which is bonkers. Evolve at level 26 so you can grab Butterfly Dance at 27, and never look back. Amount of annoying Flying-types hurts it though. Black players will need to do an in-game trade for it, so catch a Cottonnee in the park area in Castelia, go to Route 4, trade with the lady in one of the houses.
Tepig: Mid tier. He never really shines. He becomes a tank, but ingame prefers speedy creatures. That said, he's got a behemoth HP and Atk stat. Perhaps holding off on evolution might be a good idea just to get Head Smash early.
Zorua: High tier. Obtained in the mid-game, but is given to you so he gets an experience bonus. Movepool isn't shabby, and hits like a truck, but is kind of frail.
And on another note, Snivy for bottom tier. He is so godawful. I'd rather take Meganium than him.