Just back from seeing this and I loved it. I didn't enjoy Slumdog Millionaire after all the hype about it, but I loved this film. I thought it probably works best if you are aware of the amputation, it makes the whole build up to it terrifying. Every time we saw the blade I cringed because I knew what he was going to eventually use it for.
The scene itself is pretty bad, a lady in my theatre left when he was cutting his nerves. The sound effects were much more disturbing than the gore imo.
Also thought the talk show bit was great for breaking up the tension before the arm scene. "Oops!"