Here's a quick overview of some reviews from around the web. First up, it appears the i9-10900K is quite a significant performance upgrade. Compared with the previous generation, ARS Technica notes that Comet Lake-S offers a performance increase on the high end of the "what was to be expected" range. Unfortunately, it's still a 14nm part and Intel is having a though time competing with AMD's latest Ryzen lineup. While the 10900K is a couple of percentage points faster in single-threaded gaming performance, it's quite a bit slower in multi-threaded tasks and it also has much higher power consumption and heat output than the Ryzen 9 3950X. A new motherboard is required so it's not a drop-in replacement either.
AnandTech writes Comet Lake-S is a continuation of the Skylake architecture, as such Intel can't really deliver a big surprise here and we'll have to wait until the arrival of Sunny Cove to see something really new. Comet Lake-S performs well, but it's power hungry and uninspiring. Furthermore, it lacks features like PCI Express 4.0 and DDR4-3200 suppot.