Alright I'll give some comments on this, but before I say it, i have to say this in the nicest way possible: Please use some punctuation, that hurt me to read.
Now onto the real post. Kassadin is on the weak side, yes. He's not "just ok" late game, in fact, I'm really close to saying he's a hyper carry, but it's just getting to that point. The problem with Kassadin is that it takes far too long to snowball. You get killed by Fizz in lane once, you're gonna get killed a number of times after guaranteed. Kassadin first blooded you? Eh, it doesn't really matter unless he gets like 3 or 4 more kills anyways. He plays different from all of the other assassins, he goes into small skirmishes/fights, and pokes them and leaves until he has enough damage for an all in.
The number one mistake on Kassadin, unless you're going an armor mana build, is getting a tear. Don't do that. It delays your damage by WAY too long.
That being said, I've played Kassadin a lot, I've done well as him more than I have not. Do I think he's good? Sort of, but not really. Do I think he's terrible? Not terrible, but he's lower down. He could use a LITTLE bit or something, but when I say a little, I mean a little. He is on the edge of being a very annoying and solid pick if they raise his numbers a little too high. That ult buff was a LOT better than some people are making it out to be. Trust me, it's good.
By the way, don't underestimate his W. It's an amazing skill and does a ton vs melees early game if you can auto reset with it.