For shooting, I think the speed of the XQD card, and for that matter, the top rated CF cards was always only going to be important to a select group of photographers, generally wildlife and sports photographers. We're the ones who are shooting at 10fps, and shooting for more than a couple of second bursts.
I've found the XQD card's speed, coupled with the processor speed and cache size of the D4 an amazing help at capturing images of flying birds, baseball batters hitting the ball, hockey goalies stopping a shot, accidents at road races, etc.
For other photographers, there is no doubt the speed really helps out when we want to transfer photos from the cards into our computers for processing. While at Bolsa Chica a few weeks ago I was making about 2,500 to 3,000 photographs per day. The speed of the XQD card was a major help in copying the files to my laptop's external hard drives for later editing. I don't take the time to look at my photos and delete the ones I don't want while shooting. I just keep shooting and delete later when I see them on my computer screen in large size. I can better tell what to delete and what to keep that way, and I don't miss a shot because my head is looking at the monitor instead of at the scene before me.
Yes, the Series H cards are discontinued and sold out. B&H had a sale on them at the end, as the Series S was being introduced. I picked one up at a nice discount from them.
My 30sec burst testing on my D4, which I described in the thread reference above, showed I could get 157 shots taken with the Series H card in 30 seconds, with the full speed of the D4 at 10fps for 7.6sec (76 shots). That meant the Series H was marginally faster than the Lexar Pro 1000x CF card in the D4, and certainly at least statistically even.
The Series S cards are rated to have a 34.4% faster read/write speed. Once I've completed the tests, I'd find out the practical speed difference in shooting. Even if it's not much greater than the Series H for shooting, it will be a big help in uploading the images quickly to my computer.