More a complement to other answers than a full standalone one. One of the aspects which I would like to stress about those devices, apart from their ruggedness and inexpensivess, is that they exhibit a more linear behavior respect to other semiconductor devices when operated as controlled resistors.
This characteristic is fully exploited for example by Jim Williams in his design of an ultra-low distortion audio frequency Wien bridge sine oscillator (, pp. 29-32). By using a VACTEC VTL5C10 or a CLAREX CLM410 LED/photoresistor coupler instead of a 2N4338 with local feedback, he succeeds in eliminating the resistivity voltage modulation in the amplitude control feedback loop of the circuits of figures 45 and 47, removing a significant amount of distortion (compared to the already low values he achieves) from the output waveform. This is due to the fact that in photoresistors the conduction is ohmic, without any diffusive components, which are instead of paramount importance for example in JFETs (in this respect, see my answer to the question "How current is steady after pinch off voltage?").
Final considerations on the inexpensiveness of CdS/CdSe photoresistors. The main characteristic of inexpensiveness of these devices is achieved because CdS and CdSe, despite being semiconductors exactly as GaAs and GaN, need not to be grown as a monocrystalline structure for those applications. Their high basic resistivity is not affected too much by the polycrystalline structure they assume when deposed on insulating or metal substrates by sputtering, vapor deposition or other similar techniques, in order to produce photoresistors. This is a far more economic production process respect the ones required by other semiconductor photodevices.
 Jim Williams (July 1990), "Bridge Circuits Marrying Gain and Balance", Linear Technology Application Note 43, pp. 48.