The Sirui K-40X is a serious heavy ball head.
With a weight of 0.7 kg (1.54lb) and a height of 125mm (4.9″) its a bit heavier than the Manfrotto 498 RC2 but the load capacity of the K-40X is 35kg (77.2lb) rather than the 8kg (17.64lb) of the 498 RC2.
I own(ed) both but can say for me the K-40X is better to operate.
The ball head is made from aluminum materials, feels very smooth and easy in its move.
Sirui has its own locking mechanism, a little twist on the locking knob and the camera is locked as it would rest in concrete.
In the beginning I had to get used to it, as the 486RC2 was not locking that intense.
The locking head has a build in friction knob and there is seperate control knob for panning as well.
The quick release is Arca-Swiss compatible and has two bubble level build in.
I use the Sirui K-40X on the Feisol Classic Large CT-3471 Rapid tripod (my review here) and for me it is a working set.
The only downside is the one curb which makes handling difficult when need to shoot wide up,
as the locking knob moves then to the right side of the head and I need to adjust the camera with the left hand.
But this is more me, than a construction issue.
I could have bought the smaller version for 20 Euro less but I am not regretting to pay more.
All in All , I tried a lot of different Ball Heads in regards of handling, needless to say that I am very picky, I owned a lot of different heads with different mount systems.
But going back to Arca-Swiss compatible and finding this Sirui K-40X will keep me quiet for a while, I am happy as it is.