The Software Defined Network concept supposes a separate control plane- implemented as a centralized controller, to steer the forwarding (data) plane of the network nodes. The controller placement in large provider networks is still a research topic, given the various optimization criteria applicable and different network contexts. It is largely recognized today, that in large network scenarios a multi-controller solution is realistic, due to many aspects, e.g., to avoid the single point of failure in case of a single controller and to solve the limited controller processing capacity problems. This paper attempts a critical study of different placement solutions published in research studies. The main paper contribution is to identify the guidelines for a deeper evaluation of controller placement solutions performances - in future research.