I happen to love 'Mercy'. This is the return to Achtung Baby era U2 that fans always yearn for. Not to be too ethereal, but this is U2 again exploring unrequited love, their relationship with God, and the soul in a very different way than you hear on HTDAAB or ATYCLB. It just FEELS more direct, as if it is coming from a different side of U2 that we don't hear as often as we used to. Remember the song 'Wild Irish Rose'? That one is also unfinished as far as I know (maybe somewhere in their vault), but it is U2 exploring deep emotion very directly.
I would agree that the song sounds a bit unfinished and rough in a few spots, but my God, listen to the quality that IS there. I don't really care about the details - the message of the song definitely comes across and that is all I ever want out of U2. They already have shiny, perfectly produced songs out there, like Vertigo and Beautiful Day.
'Mercy' allows us to enjoy the U2 that created songs like 'So Cruel' and 'Wild Horses'. Raw, vibrant, deep, soulful stuff that sticks in your heart and lingers.
AJ