I am tempted to move up from my 9900K to the 12900K, the (supposed) 40-50% IPC increase over my 9900K is a BIG deal for what I use my PC for and would make a real difference, but I'm also interested in these new 3D V-Cache Ryzen CPU's coming early next year. Alder Lake is a first generation platform with all the trouble that usually means for the early adopters whilst Ryzen is now a stable mature platform, a 5900/5950 with the new cache feature will, I'm sure, be in the same ballpark performance wise, with none of the problems of early Alder Lake Adoption.
I certainly won't make a decision until the new year with Alder Lake though until I've seen if there are any horror stories with them and the new Motherboards..