I played against a chogath recently (in coop vs ai) and i noticed how important his buffs were. Most noticeable, his Rupture (aoe knockup) is way faster, making it much harder to avoid. I find this sudden buff (not just that but all his other recent buffs) kinda suspicious when you realize they are releasing a legendary skin for him.
They are indeed INCREDIBLY powerful. I noticed just how great they were when I recently played him. The rupture is actually aimable instead of simply zoning with it. The silence speed-up is almost unnoticeable, but I don't mind. The vorpal spikes is ridiculous range now. Which is awesome, because he really was underpowered as a tank or an AP. All in all, Chogath is now a great addition to a team.
I don't believe that it was just the buffs. In my opinion, it just called attention to how powerful Cho'Gath is and has been. I played Cho'Gath extensively even before the buffs and even after the targeting circle was added to his Rupture. He's been strong, especially against common mid champs like Karthus and Ahri. He has the natural sustain of a bruiser, but the poke and burst of an AP champ. At worst (so far), my mid lane with Cho is a stalemate. At best, I enter the mid game with 3 or more stacks. When I get Deathcap and a bit more AP, my ultimate suddenly bursts more true damage than the average Darius (possibly an exaggeration, but with AP, Cho's ultimate does ~700-900 true damage on a 40-60 second cooldown.)
Cho's always been strong, just undervalued in teamfights because of mildly long cooldowns and an inability to initiate properly. I intend to pick up the Battlecast Prime skin. He works well enough as counter-initiation, though, and God forbid he lands Rupture/Scream/Feast on your carry. Honestly, though, I'm glad for the buffs.
Although, looking at the replies so far, apparently, he was just strong for me, but I also enjoy playing Brand, who has a similar ability that's important to his kit.