What is the value of x-ray inspection of an assembled PCB?
I suppose it would be quite valuable for examination of multi-layer boards.  

There are less obvious uses, some of which appear HERE.  The bottom line is that Xrays are better than optical techniques at assessing the quality of a solder joint beyond presence or absence.  How important this is for any given application, given the higher cost of the imaging modality, comes down to failure-mode analysis, Pareto analysis, etc.