I would say that DC/OS is k8s competitor. It is trying to achieve the same orchestration level as k8s using Mesos as underlying process (container) orchestrator (: So.. not sure why you want to run k8s on it.
Majority of DC/OS users are running distributed data services like Spark, Kafka, Elastic, Cassandra, etc. And they're doing this for at least three reasons 1) Pooling these distributed systems instead of running them in separate clusters, 2) The automation for scaling and high availability that Mesos frameworks enable, and 3) Being able to run these data services in any cloud they choose - Google, Azure, AWS, or their own. The 2016 Mesos Survey (n=500) has more details.
If you are squarely focused on container orchestration, 62% of DC/OS and Mesos community runs containers in production. Average size of their environments is 200+ nodes. A similar survey of the K8s community showed rough average K8s environment is around 20 nodes.
Disclosure: I work at Mesosphere
https://mesosphere.com/blog/2016/11/02/apache-mesos-survey-2...
http://blog.kubernetes.io/2017/01/kubernetes-ux-survey-infog...
For most people k8s (like any other distributed system) is pretty hard to operate and wrap their head around. The goal of DC/OS is to make building and operating distributed systems easy. Mesos is pretty different from k8s. It doesn't have an API for microservice developers for example, instead it has low level primitives similar to an operating system kernel, on which you can build any software. We recently launched an SDK to make that even easier, along with a bunch of open source and commercial software that uses it, for example Cassandra/Datastax, Kafka/Confluent, Redis, Elastic, Couchbase, Alluxio. So DC/OS ends up looking a lot more like an entire cloud platform like GCP, which offers managed k8s, a managed database (BigTable), machine learning and so on. Under the hood it all runs on Google's proprietary Borg, which has some similarities with Mesos (Google is a sponsor of the lab where Mesos was created).