Why does Madara appear unhurt despite being covered in wounds?
From chapter 659,

Madara mentions that due to the healing powers of Hashirama's he is able to recover must faster and is taking close to no damage.
So it is not just just Hashirama's senjutsu, but also his healing powers that have made Madara virtually invicible.
Also from the same chapter:

It is not that he is not being damaged, but that his healing is so fast that all damage seems to have no effect at all.