As previous reviewers have said, the Joby gorilla pod SLR Zoom is more or less useless without a ball head - and chances are most will consider Joby's own solution to the problem. It's competitively priced compared to other options and performs just as well as more expensive heads.
The Gorillapod head is a simple design, a metal socket, encasing a solid metal ball which can be fixed in position by turning a wheel.
The head is well built and solid, with smooth action when the ball is unlocked and a firm locking mechanism which gives the user confidence it will hold the advertised 3kg load limit. It can also be moved into a side slot to give a portrait orientation.
I've used it with a Nikon D300 and 70-300 zoom lens without a problem and with no sign of creep from the head itself (although the gorillapod legs themselves caused some motion - see my separate review).
The mounting plate has a built in level, again an essential when using with the hard-to-level gorillapod legs. It's disappointing to see just a single plane level, rather than a much more useful round bubble level which would let you level the camera in all directions.
Like the Gorillapod itself, the head might not be perfect and photographers need to know it's limitations - but otherwise is a good match.