Used Sennheisers with ANR in KingAirs for a long time. I would never fly a TP without it. The real issue is the long term impact on your ears by higher than necessary noise levels AND fatigue generated by that noise. I nowadays fly mainly a C680 which is not too noisy in the office, nether the less I notice a difference in fatigue on long days with ANR on vs. off. IMO the A/C manufacturers should provide ANR headsets as a standard or at least a standarized power socket...
I sometimes fly a B200 equipped with Bose ANR headsets which do have an annoying sound generated obviously by the ANR electronics. Since I can`t compare this set with another Bose I don`t know wether its specific to that single set....