Usually it's as happy as... rather than happier than... when I've heard the expression - tinman's version, that is. Is this an example of American competiveness?
I've heard "happy as a house cat" which has a nice ring to it. Or "as happy as a child with a new toy."
Also, I once heard of a sexually inexperienced young man who had just wed as being "as happy as a child with three new toys" but that's another thread entirely.