I suspect it will be more bittersweet than anything.
Yeah, Martin has said in the past that the series' ending will be bittersweet like the ending of LotR.
Lord of the Rings is one, massive, happy orgy at the end. Yeah, there's some sadness, but it's completely buffered by long, fulfilling, prestigious lives for most of the characters. If Song of Ice and Fire is like that, nobody in their right mind will buy it.
edit: Buy as in accept it as a reasonable conclusion to the story, not purchase. It'll sell a shit ton even if he decides mickey mouse gets pooped out of a dragon and builds a castle on the bottom of the sea to rule the entire world.