I have a 90 degree prism and WLF viewfinder for my hasselblad 501c. I never use the prism. First of all the camera is lighter and more compact. Also like others have mentioned, with the WLF it helps my composition. It's only psychological, but it does make a difference to me. When I look down on it, to me it looks like I'm watching a photo and I'm much more likely to reject and correct photos that I would normally take with a prism.
As for the mirrored image and panning etc, I find it quite easy. I just hold the centre of the camera in my left hand, while moving the back of the camera in the direction the object is traveling (or is) in the viewfinder. The lens will follow the correct direction. But it sometimes can become bit difficult on diagonals. If I get all confused, I look up the scene again and back at the viewfinder. It will become second nature after few days.
Also my Acute-Matte screen is very bright even on the brightest and sunniest days in Australia. So the stray light was never an issue.