I fly electric powered planes with Lipos and have had great results. However, I wouldn't use these batteries for the receivers in my glow planes unless I had an extraordinary need for either weight reduction or long, long duration.
The lipos do not have a well documented history for length of service or reliability. They have special charging requirements and can be destroyed through overcharging or discharging at too high of a rate. Failure mode can include a fierce and difficult to extinguish metal fire (remember NASCAR driver Robby Gordon's car bursting into flames while he was leading at Watkins Glen in
2001? - Lipo battery failure in his TV telemetry box). The battery manufacturers recommend charging in a fireproof box (not in a valuable aircraft). They need a voltage regulator circuit for use as receiver power (available from FMA Direct) - another potential point of failure.
NiCads can be used, abused, fast charged at the field in less than 20 minutes and still provide years of service. If you need to save some weight or increase electron supply without gaining weight, then go to NiMh cells and take extra care with charging and monitoring, because they are less robust than NiCads and will not last as long.
Lithium Polymer batteries have opened up new possibilities for electric powered flight. Small electric planes with Lipos and brushless motors can now easily outperform glow planes on a power/weight basis. Planes that will hardly get off the ground and fly for two minutes on NiCads can exhibit sparkling performance and duration on lipolys. That said, I beleive that the extra cost and special care and feeding requirements, along with questionable lifetime, make them an inferior choice for receiver power on glow powered or sailplanes.