You can easily view To-dos for other users simply by switching users from one of two places:
On the Home screen you may click on any user's avatar and it will bring you to their work stream page. Then click on Features and To-Dos. From your My Work screen, simply select Switch users (upper right corner) and select the user for which you'd like to review To-Dos.
If a To-do is not associated with a product then only the creator of the To-dos and user it is assigned to can see the To-dos.
A note about permissions:
Most permissions in Aha! are derived from the products and product lines. As a user, if you have permission to view a product then you will also be able to view all of the To-dos associated with records in that product. The ability to switch which user's To-dos you are viewing doesn't circumvent this permission.
Aha! does not yet have a concept of a manager who can view a subordinate's To-dos, so we err on the side of caution and prevent cross-user viewing of To-dos.