For the context, I am a mastery 6 AP Kog'Maw main, though I also played him ap supp (I don't recommend that without experience)
I already played him a lot before the rework but I started maining him afterwards
the "new" ultimate really allows for a more complex play style. although I really liked the old ap kog, I think the old ult was too monotonous.
For those saying that the new ult is useless, I'll let you know that when the target has <50% HP you will always deal a higher damage than before.
You now have to focus on low health targets, and it is NOT restricted to squishies. Please also notice that you will sure kill any target below 25%HP, sometimes even tanks.
The reworked ultimate offers, in my opinion, a more "competitive" type of play, where you need to take more care for your mana than before (each shot costs 50 mana increasing instead of 40), and where you need to choose your targets better, and know how to make them targetable
Please don't revert the changes to Kog'Maw's ultimate, at least if you consider the ap build.