Based on our current state of knowledge and ability to interpret genomic variation, good candidates for WES are those with suspected monogenic diseases. In some cases, these patients can be diagnosed by sequencing a single gene.
Other conditions, however, are genetically heterogeneous (i.e. where mutations in more than one gene can cause the same clinical phenotype). Examples would include: childhood onset epilepsy, muscle disease, severe combined immune deficiency, intellectual disability, family cancer syndromes, and cardiomyopathies.
Often these cases are better approached by WES rather than proceeding gene by gene with Sanger sequencing. Other candidates include patients with severe sporadic disease, particularly early onset cases, where de novo mutations may be responsible.