I have grips for all my cameras but i only put them on if I will be shooting a lot of vertical
In general I prefer to go gripless - easier to store and carry
The 6d grip is one of the better hand fitting grips. The older grips for the 20d-50d, 7d are not comfortable
Also I think the 5d3 grip is the first one with the joystick built in - did not see this for other cameras.
I use the joystick a lot so its important the grip has it.