How Does Your Org Use Planner?

It's a challenging one, and with the product really on the market for 4 months there's still a lot of tweaks coming down the line that will help with user adoption.

While everyone works differently, my advice to people is to keep tasks.office.com open (as well as Yammer, but that's for other reasons) for the majority of their day so they can quickly and easily see what they have on their plate.

Unfortunately we can't force people to be subscribed to notifications, so it's an overall adoption thing of working together as a team and regularly checking the tool they work with.

I know that's not an ideal answer to your question, and the immediate user resistance will come from having to open another page/app where they already have a bunch open. Unfortunately I've found this is the best approach though - to make people want to keep on top of it.