Despite me never playing Kog'Maw, I can say that I don't like this rework. Before rework I considered buying Kog'Maw and starting playing him both AD and AP. I saw AD Kog'Maw as an armor shredding high damage immobile hyper carry. Besides BotrK, Kog'Maw didn't build any on hit effect items, he built for armor shredding and sheen procing from Trinity. While rework made him more of an immobile hyper carry, I feel like it took him from armor shredding ADC to an on-hit monster. Maybe some people like this change, but this change just discouraged me from trying out Koggy.
Then, there's AP Kog'Maw, the reason I was considering trying out Koggy in the first place: I found it cool that Kog has 2 completely viable ways of playstyle - either hyper carry or poke mage. His ult made him great sieger. Making it an execute was... imo dumb. It just took away a lot of AP Kog's power while being not so crucial change for AD Kog.
Personally, I don't like this rework, it just completely discouraged me from ever trying out this champion + killed his diversive build paths. It just makes me question: why he was changed in the first place? He was perfectly fine, maybe he didn't have very high pick rate, but that's because there isn't enough team communication in solo queue for him to work. On the other hand, he was very good pick in 5v5 and saw some LCS spotlight from time to time. Right now, he is even worse in solo queue, because he NEEDS to stand still in order to maximize his DPS potential, so he will die pretty fast, and at the same time, picking Kog'Maw in 5v5/LCS means your team comp must be focused on protecting Kog even more than before.