If you don't shoot sports, and don't shoot full-burst, and have a camera with a good buffer, and do not shoot lengthy, high-rez video, then it seems to me that it is not economically sensible to pay the huge price premium for XQD cards. Just.Not.Worth.It.For.Most.People.
I realize most of this information is out of date, but this is the largest database that I know of of actual, real-world memory card performance speeds and timings, as related to commonly-understood d-slr models and commonly-understood card types and speed ratings.
Rob Galbraith DPI: CF/SD/XQD Performance Database
You're a smart guy, and you other half has a high-end Nikon or two, with known buffer depths and frame rates. Using the data provided on Rob's site, you can perhaps draw some conclusions, and can see the actual,real-world frame counts and buffer clear times and card-write times.
I think that, for some situations, the XQD card _could_ be worth the potential speed in emptying the buffer timings, for things like say,extended action sequences, or bird-in-flight or other nature type scenarios, where you might get _one,single opportunity_ in say a day or a week, to capture a long sequence that could never be repeated. Otherwise, you camera's 29-frame-or-more RAW buffer ought to more than adequate. What is the real killer though is when something extraordinary occurs, and you shoot a buffer filled with raw images from a new,high-megapixel sensor, and it takes too long for the NEFs to flush from the buffer and be written to the card, and you stand there almost helplessly as your formerly high-speed camera is able to do nothing but click off a frame every three seconds or more in-between clicks, and y
ou watch as once-in-a-lifetime shot opportunities go down the toilet....and THAT is when the five times the price cost of the XQD card justifies its cash cost...but, those types of scenarios do not happen for most of us on a regular basis...but still, when they do...
And of course, there's the video side of things, where the fast cards perform better. And there's also the download transfer speed side of things, where if you have a high-speed card reader, the faster off-loading times of the fastest cards can make downloading files to the computer less of a time suck.