We've tried and then stopped using Planner at least 3 times in our organisation - we've been impressed with the progress but each time been limited by a few key features.
We really like the integration with To-do as this solves one of our concerns that an individual couldn't easily prioritise their tasks from different plans. Although there are still some different features between Planner Tasks and To-do Tasks e.g. start date, priority
One of the key concerns I've heard from my colleagues and clients I work with is: A team member may be a member of several plans and a manager would like to see how busy they are across different plans to decide whether to assign them a task. Is there a way for a manager to see the tasks assign for each member?