This ball head is very similiar to it's now discontinued predecessor, the B-1 (in it's day, considered by many to be the best ball head on the market). The basic quality and functionality of the Z-1 is, for all intents and purposes, identical to the B-1, but there are a few changes worth mentioning.
What hasn't changed;
The Z-1 is relatively light-weight (1 lb 8oz) and it will hold a tremendous amount of weight for its size. You can use this head for lenses as large as 600mm f/4 (some people do not recommend using a lens as large as a 600mm f/4 on the Z-1, but it will hold the weight). The smooth action of the ball is also quite remarkable. It exhibits almost no “stick-slip” or jerky action when tension is applied to the ball. This makes for easy framing adjustments. The action of the panning base is also quite smooth. The distance between the center of the ball and the plate-support surface of the clamp is 1.72”.
Slightly Lighter: The Z-1 is also about 2 oz lighter than the B-1 (1lb 8oz vs. 1lb 10oz).
Pan-lock knob change: The only other noticible difference is the shape of the pan-lock knob and the look of the panning base. The Z-1's pan-lock knob is a small teardrop-shaped lever rather than the small knurled knob found on the B-1. The new design makes it possible to lock the the pan mechanism with more force, which is particularly useful for installing and removing the head from your tripod. However, in my opinion, the lever is a little more awkward to use, particularly, in conjunction with tripods with a large diameter top plate (e.g. 3-Series Gitzos), in which case the lever hits the top of the tripod. However, it is quite possible that after using the Z-1 for a longer period of time, much of this awkwardness would wear off.
Tic-marks on the pan base have a new look:
The panning base of the Z-1 looks a little different than that of the B-1. To me, the rounded lip, yellow pin-stripe, and hidden panoramic tic-marks on the B-1's pan base looked a little more refined than the Z-1's tic-marked silver band, but that's just my opinion, and I think Z-1's tic-marks are more functional than those on the B-1.