I started by spinning up a small one in my lab but when I saw the pricing I back-pedalled very, very fast. Deleted the whole Resource Group and never looked into it again.
But also, every time I start stringing together cloud services, I experience two things: first, exploding complexity because now I'm adding points of failure, integrations, transformations, to keep it all running; and second, this sense of "the whole point of the cloud is to simplify things, to offer canned services and features that save me the trouble of doing this in code for myself." Once I'm using cloud features to work around cloud limitations, I bail out because if I'm going to spend that time (and money), I'm going to get the benefits of something much more direct.
So I tried setting up the same ingestion database with Kafka Connect and Mongo on a $200/month VM. It worked flawlessly, and Azure helpfully suggested I downsize that VM because it was underutilized based on CPU statistics.
What incentive do the Cosmos engineers have make it more efficient, or to make the RU pricing model more reflective of actual usage? Zero. It's a money hose. Why would you turn that off?