I agree on the 'it depends' front! I have done quite a few part-pay flights. First was when they did a 30k Avios = £500 off promo combined with an exEU in J to SFO which was amazing value making the flight £750pp. Have also used it on some positioning flights for a TP run where the overall run produced a similar amount of Avios making is Avios-neutral. Finally I used it to reduce the cost of a jaunt to LAS at Easter which makes my CPH-LHR-MAN-PHL-LAS-JFK-LHR-CPH ticket about £250 in Y which is very cheap although not luxurious.

In all cases, the p/Avios was not important to me at the time so it was worth it based on the different circumstances.