Microsoft Planner is a tool that is included with most Office 365 subscriptions. It's often compared to the popular Asana and Trello tools. Each has their own benefits and weaknesses. At the end of the day, like any software tool, you have to be comfortable with it and find it relatively easy to work with in order to get the benefit.
One benefit is that Planner is included in the cost of most Office 365 subscriptions, so you may already have it and not know it. If that's the case, check your available apps to see if Planner is there. Be sure you click on All Apps, to be sure you are seeing everything you have access to.
So what exactly is Microsoft Planner? Microsoft's calls Planner a Task Management Software. More specifically, team based task management organized into plans and buckets to visually organize your work, share tasks, files and interactively chat with teammates working with you. If you are in to the latest trends, you may think of Planner as a Kanban style tool, albeit a lite version.
Some say you should not consider Planner a project management tool, but others make the case that it can handle lite project management. Certainly coupled together with other Office 365 tools, you can build out a project management process that may work for you. That said, the built in tools will not provide you with the right project and resource management tools that a dedicated project management app will.
Personally, I like Planner and I like where it's heading. It integrates well with other popular tools like Outlook and Sharepoint and ties in nicely with many more. There is also a growing community of developers creating add-ins and utilities to improve workflow between various applications and Planner. I also like that it works cross platform and device, so you always have access.
While tools like Asana and Trello are more mature, if you work within Office 365 you should check out Planner and see if it will work for you and your team. Like most Microsoft products that have come before it, Planner may be a bit late to the game, but it's catching up quickly and likely to become a major application in the area of Kanban style collaborative tools. Check it out and let me know what you think.