He feels meh to me after his nerf fiesta. He was certainly op, but now he feels bad- I think the ridiculous 1 second delay on his ult should be removed so it can actually become a useful ability. Maybe change the q width to 80 from 70 and give the ult passive ratio a 1% increase at each rank so it becomes 3/5/7.
Source: 100k mastery Tahm Kench player
I think he's plenty viable right now. His utility is impossible to replace, and his scaling from health is a cut-n-dry double-edged sword.
honestly if they make his ult 3/5/7% and get rid of the 1 second delay on it, he would be in a good spot again. new mage itemization really helped him out though in 6.9, try building 
. you get a huge health pool, lots of dmg, max cdr, tons of dmg, ap and lots of hp regen. i got like 200k mastery, almost lvl 7 on him.
