Me for starters, used all the time. All sports and wildlife photographers use that speed. I can recall doing a golf swing sequence with Tiger Woods a few years back and even at 1/8000 with 10 fps then (quicker now at 14 max), it wasn't ideal.
I've got a 1/8000th camera or two (1Ds Mark II certainly, and i'm also guessing my Eos 3 does as well). Can't say i've had much use, other than avoiding ND filters when shooting big aperture stuff outdoors.
Sounds like a fun sequence. I'm guessing you'd see artificial bend in the swinging club as the shutter actually takes 1/250th to move across the frame (or whatever the flash sync speed is on the new canons), but only in the horizontal direction.
As for the shared battery and card compartment, this is actually one thing that does annoy me, but only if the cover fouls on a grip or tripod mount, but it's obviously not something that would cause me to reconsider the purchase!
My Mamiya ZD has a continuous mode, and shoots 1.1 seconds per frame, for a burst of something like 10 frames. After that you have to wait a minute for it to flush to disk ;-)
Joe, which colour one do you have on order?
Cesare