We've been studying some geometry lately. My math child is never content to do anything just like the books says, and when he saw his Saxon book had an activity sheet with geometric shapes he was running upstairs to get the tangram set from its dusty spot on the bookshelf. (We haven't used them since last year at this time!)
What followed was more than an hour of working through the cards in this set. He methodically went through each one, until he had completed every single card.
While my daughter was laboring over a Saxon 7/6 math test she was watching him out of the corner of her eye. Rather than play with the tangram set he was using, when she finished her test she wanted to find some online tangrams. Fine with me -- I'm all about letting my children's interests lead them at certain points in time.
I'm not sure of the documented benefits of tangrams, but here's what I observed in both of my children:
How much better it is to physically manipulate shapes than just to read about them in a book or draw them on a piece of paper! Tangrams are the perfect actiivity to reinforce your geometry studies, or just to do for FUN.