The XF4 one was great because they at least sang Christmas songs and the duets actually had some interaction with them. Plus it was on one night for a change, none of this "two night, four acts" rubbish.
XF5 had an amazing duet (Beyonce/Alex) and generally good performances all round by JLS and Alexandra. Plus the series as a whole had been superb and I doubt that series will ever be matched in terms of talent.
XF6 was a big step down but again, the duets weren't bad and the performances all round were decent enough.
This year, though? Three dreadful duets, completely wrong performance order (Cher and Will.I.Am last over Rebecca/Xtina and Matt/Rihanna?), dreary atmosphere....