I really like the waist level finder on my 6x6 Bronica. The WLF allows shooting from much lower than eye level; thus avoiding all pictures having that 'looking down' signature. There's the flip-up magnifier that makes critical focusing possible. Another plus is the size/weight/compactness advantage over a prism; on 6x6 these are big. The left-right reversed image has it ups/downs; on the one hand, seeing the world reversed can help judge composition better, but on the other, it makes for a weird experience getting your prints back.. they really look different from what you saw in the finder.
Obviously, on a 6x6 you don't need to tilt the camera sideways for portrait/landscape orientation, so a WLF is no hindrance in that respect. For a 6x9 I'd probably regard it in a much different light.