I haven't used either King or Hadley hubs, but I have a set of I9 wheels. I'm happy with them - I have been able to maintain them using standard tools which is a bonus. The construction of them is amazing, fantastic machining, a real work of art. I have had to replace the axle due to the previous owner cross threading the bearing preload cap. Dealt with I9 directly in the states and they were fantastic.
I'm not sure if I noticed the extra engagement points, but I no longer get the cracking sound that the bontrager hubs used to make. I did find that getting the preload on the bearings took a couple of goes, but probably no worse than the old cup and cone style. They use standard bearings which I should be able to buy at the local bearing shop. What more can I say? they work and I haven't had any problems aside from the axle which was operator error.
My hubs use the proprietry straight pull aluminium spokes. I have noticed that some of the heads of the spokes are corroded and will need replacing - they are laced to DT swiss rims with stainless eyelets, so I am assuming they are corroding due to dissimilar metals being in contact. If you don't want to use their spokes, they do make hubs to suit standard spokes.
I'd be keen to know who the distributor is in Australia now, I need a couple of spare spokes