Is karthus still still viable or good?


Short answer: yes. Long answer: yes he is.
If no one plays him in LCS in the next month, no. (By the community's standards).
Every champion is viable. The only champion I'd tell you wasn't viable is poppy because she's cancer and no one should play her
i don't think karthus is good at all, last time i played karthus went to the end game i had full build and 1 vs 1 against enemy khazix (prob also full build). he went in, i exhausted him right away, turned on pool, q spam with 80% accuracy. and he killed me, i q some more, ulted as well. and he still lived after that. when a mid laner lose to jungler 1 vs 1 with full buiid you know it sucks. btw incase you wondering i always build karthus the same roa Seraph's Embrace (i used it) deathcap, zyonya (don't remember if i used this against khazix or not), void staff, and boots. don't remember if i had the ap elixer either. the fact that he killed me and walked away even after i exhausted nearly everything is a no brainer how shit karthus as a champ is.
SHHHHH!!!!!
Dont remind riot he exists!
They might "Compensate" him for dfg or some shit!!!
I am no expert on Karthus however I believe he is still pretty strong
He scales well and can be incredibly strong in teamfights doing huge amounts of damage and even if you focus and kill him he will still deal lots of damage (especially if your allies can keep the fight on top of your passive/corpse)
however he is a rather passive champion who mostly just sit and farm trying to scale and stronger... and mostly just assisting by using his ult and his laning phase isn't the best (he isn't bad though as he does deal good damage if you are able to land Q's on the enemy however he is often squishy and vulnerable given that he has no mobility)
oh and he is also pretty good at taking baron/dragon as his Q works pretty well at them
yea he's pretty good however his strength lies in his E if someone like lux or tf misses there skill shot on you, then you can engage on them with your E and spam Q well doing so thus why exhaust and ghost are better for him then flash ignite, unless of course your doing some sort of homegaurd teleport karthus gank, which although silly is effective but like swain you need to not pull the plug on your mana, so its either to all in or to do severe poke because they have no way of fighting it c: 
Not sure, haven't tested him - won't play him until Revive comes back.
Yeah, he is amazing in teamfights and can be a support powerhouse. All you have to do in the early game is survive and farm until it comes time to teamfight, then reck and repeat.
You mean penta steal karthus? Yes
I like karthus. I haven't been able to do well with him in a while. I feel like his E does no dmg late game
He's a good range-champ. So take him if you want poke. And he can pick up a ton of assists with his ult.
Apparently the highest (or one of) winrate when mained over 125+ games, higher than riven even. So yes?
Yes. Just not AS GOOD in the tank meta. When playing against a bunch of squishy assassins, he had more than enough time to dish out tons of damage and his ultimate did more damage as a % of total health.
Now that 6k HP
and 5k HP
and 4k HP
with a 1k HP shield can sustain through his damage pretty easily.
So yeah, still strong. Just not as good of a pick compared to when everyone was running squishy assassins instead of beefy tanks.
I've played Karthus. Late-game, you 3-shot squishies from range. If they're alone anyway. And if you just barely miss... ULTI!!
hes free
Dunno how he's doing in other modes, but in ARAM he's more than fine. Especially with mark-dash spell :)
Yes. Just very difficult to use and vulnerable to people who can jump on him, and also takes FOREVER to scale up his AP and mana pool, but it's worth the wait, IF you can get there.
It's the same problem with 
He's better and more viable as a late pick. If the other team decides to go super shield on you then your game will probably be very frustrating.
The key to Karth is maximum melting. Get your
by 20-22 minutes. Spell vamp is essential
. Spend nothing on armor or MR. You WANT people to come at you. Death is no big deal. Avoid

The only real option with his current kit depends on your confidence.
is the item for when you feel confident.
and
for a more conservative approach.
for very rare games with many tanks.
His best end game in almost all situations is:

Flashing into the other team with your
shield on and
ready is the move to break logjams at towers. It's a base breaker. It requires very little if any help from your team.
Hope that helped! I've been having success with it recently.