I have them on both my bodies. With my large hands, I find even a large camera like the 5D2 only sits in part of my right hand - only three of my fingers comfortably fit on the grip, and a large part of my palm isn't in contact with anything. With the grip, all my hand has something to grip onto. Even though the camera weighs more with the grip, the extra stability the grip gives makes it easier to handle. With larger lenses such as the 70-200 II, it makes a world of difference.
Other advantages include vertical shooting is greatly simplified, the ability to use AA batteries should you end up on a long trip away from the mains (or if you want to pack a set of cheap duracells for backup at an event), and also a worry free way of putting the camera down on whatever surface you want, such as rocks. This means only the grip and lens hood are likely to take the brunt of your carelessness - the cheaper to replace parts of the camera/lens system.