Although there is the Add Geometry Attributes tool, what happens if you want to calculate the geometry (X and Y) in an existing field. The Add Geometry Attributes tool creates new fields for this.
You could use the !shape.extent.XMax! , however, this takes for spatial reference of the dataset. What if you want to calculate decimal degree coordinates from a feature class the has a projection (or visa versa - calculate meters from a geographic coordinate system)?