It was horrifyingly good. There was a brutal sense of irony when Walter is claiming that his son will stop him from being a cruel man... and the son had already done so. It reminded me of Hamlet - everyone knows it's a tragedy - and I couldn't stop watching the slow descent of both Walter/Peter.
I guess that was the crux of the whole fighting against the Observers - if one has to win, then one has to be an Observer. However, if one does become an Observer, all shreds of humanity is lost. If one wants to lose ... well that is losing. It's a Catch 22.