According to their announcement of Product Sans and their new logo, the typeface was supposed to be used in promotional materials and lockups, but there’s no mention of it being used for product UIs. In fact, the only other product I can find that has this same inconsistent mix is the new Gmail.com, also previewed today.
It isn’t just about what these typefaces look like, either, but how they’re used. For example, when entering a new task, the name of the task is set in Product Sans; when it is added to the list, it becomes Roboto. Tapping on the task takes you to a details view where, now, the name of the task is in Product Sans. There are three options to add more information: if you want to add details, you’ll do it in Roboto, but adding a due date will be in Product Sans. The “add subtasks” button — well, text in the same grey as everything else except other buttons that are blue — is set in Product Sans, but the tasks are set in Roboto.PixelEnvy: Google Debuts a Standalone To-Do App, Google Tasks
I haven't bothered trying Google's todo-app because, well mainly, you know, it's just another tool but I did try the new Gmail. After using it for less than 5 minutes, I can only pray that a) Google lets users keep the old version for a long time and b) that a new version of Apple Mail is on the horizon. Because the new Gmail is terrible and by the sound of it, so is Google Tasks.