I got a PB-E2 to use with my EOS 3, mostly to use it easier in portrait-orientation, not for the 10fps (I rarely even shoot high-speed on digital, I don't particularly want to be changing film rolls every 4 seconds).
I have some RSI problems in my right wrist, mostly computer/work related, and a skiing accident a few years ago busted my right shoulder so I can't hold heavy weights above my head for too long. So I thought a grip would be the perfect solution, I could hold the camera the 'normal' way in portrait orientation, especially when I can't stabilise it with my left hand because I'm using a 'smaller' lens (mostly Tak 50/1.4, FL 55/1.2, EF 85/1.8, EF 100/2.0 in the 'portrait' range).
But it's actually a lot harder to use than I'd thought, because when it's rotated the eyepiece is actually halfway down the left edge and the shutter is above my right ear. So I've barely used it, it's no better (and possibly a bit worse) than just holding the ungripped 3 rotated shutter-over (sometimes when my arm gets sore I'll swap to shutter-under, but I can't do it for too long and can't be as soft pressing the shutter button). One day I think I'll just on-sell it (although it's a nice AA-holding backup, so maybe I won't).