I voted as not happy with PC,... BUT
My primary machine for accessing mobileme is my iPhone.
My secondary way of accessing mobileme are the webapps, using Firefox3.
All issues are, as far as I can tell, due to poor implementation by Apple. I believe all the problems have to do, not with specific platforms, but with the MobileMe core services themselves. It might be easy to blame one platform or another, but when you have inconsistencies between the webapps themselves, then that can only be blamed on an error in MobileMe's Core Services.
That's why as orleansMac points out, MobileMe stinks for both.