One is an online service and the other is an operating system...
... and as such cannot be compared.
One user may find Windows Vista to have no problems at all, some may have minor issues and others will have major problems - such is the complexity of an operating system.
MobileMe on the other hand is a service, so if something goes wrong or there is an inherent problem with the service, then it's likely that more people will experience issues. (e.g. when me.com didn't work when it launched, nobody could use it).