Aguilera actually pulled "Mother" off pretty well. But Avril doing "Imagine" is...a joke, to be friendly. I don't even like the song but Avril just butchers it. And R.E.M. sucked the life out of "# 9 Dream". And the fact that The Black Eyed Peas make music in general is enough of a crime, but to let them touch Lennon is unforgivable.
If they wanted to do this properly, they should've gotten guys who actually knew John (Paul, Ringo, Elton John (who did a great version of "Imagine" back in 1980), Clapton, etc.) to do it.