I see there are large number of grid frameworks in CSS like 960, heroku grid, etc being used by huge websites. I want to know whether using grid structure is really useful? If yes, then how?

One of the biggest problem I saw with grid is having equal heights for elements. If we are using three grids like grid_2, grid_7, grid_3 for 3 vertical panels then it becomes very difficult to have these three panels positioned in a way such that they have equal heights and all of them change height when any of the content exapnds or collapse.

This is because elements are floated in grid system and they don't change height along with neighbouring element.