I've used a lot of high-end ball heads for my work, but sadly not this one. It's never made it to the top of the list, being a bit specialist and expensive. It's not Gitzo's most popular head, and good reviews are hard to find.
Gitzo ball heads are generally excellent but have not really caught on because of Gitzo's unfortunate quick-release system. Rather than bother to convert them to Arca-Swiss, it's just easier to buy something else, but now Gitzo also offers an Arca-Swiss option. Hoorah
I feel fairly confident in saying performance will be excellent. Everything points to it, from the huge ball, the weight (getting heavy), the high cost and Gitzo's reputation. It also holds true to the Systrematic's low centre of gravity design that puts the camera in the most stable position. A couple of obvious downsides are the limited tilt angle and, without a drop-slot, you need an L-bracket to turn the camera vertical.
The other question mark is over the ergonomics and handling. It's different, though that's not necessarily a bad thing at all. I suspect it's a bit Marmite. You'd need to try one.