We built all of our machine learning backend in D at AdRoll. Some examples:

- learning of large-scale classifiers and regressors using custom optimizers

- real-time pricing of billions of ads a day using these models on ad exchanges. <.5ms latency to parse complex bid requests and compute sparse and simd dense dot-products

- a real-time event processing system that hits DynamoDB with ~4.5K json queries per sec on a single node

We literally have D systems deployed on hundreds of ec2 instances as we speak and responsible for mission critical tasks of a >100$M run rate company. D is ready for prime time and works at scale.