I actually read somewhere that this song was the result of a fight while recording. Thom got really frustrated with their guitarist and punched him when he kept messing up. They then decided to slow the results of the fight down and turned it into what is now Treefingers. I could be wrong, though.
I was into ambient music before I got into Radiohead (I started listening to them because I heard they cited Aphex Twin as an influence), so this wasn't too alien for me, I thought Kid A as a whole was fairly conventional. That being said, this is a great ambient piece,
Every good album has a song that's a surprise. This was Kid A's surprise track, and despite repeated listenings over the years, this still sustains that quality of surprise for me. Love it. (also, ever so much more awesome with the right headphones)