For me the jury's still out on ML.
I'm still using Snow Leopard because of:
#1 Loss of grid based Spaces. It's sad that a 3rd party, Total Spaces, is having to bring this back and not Apple.
#2 Lion's Full-Screen mode with a 2nd monitor. In SL, I can have a video in Full-Screen mode on my 2nd monitor, and either still access the desktop, or have a 2nd app in Full-Screen mode on my primary monitor. Can't do that in Lion and apparently not ML.
#3 Expose seems to have only partially returned in ML. Mission Control doesn't seem to allow as much freedom of movement of windows between desktops as SL allowed with Expose/Spaces.
#4 "Save As..." and "Do you want to Save?" doesn't seem to be back completely.
#5 Limited control over Resume, Versions and AutoSave.
ML does seem like an improvement over Lion, but it's still missing abilities SL. Thus, no plans to change from SL in the future.