PIP has been available to new claimants since June 2013. Most of those who are currently on DLA will not go through the reassessment process until October 2015. However those whose award runs out before that date or whose circumstances change will still be called in, potentially piling delay upon delay.
When it's their word against yours, medical evidence is what proves you're not the one lying. Medical evidence is what says that you are in pain, or you are blind, or you are mentally ill. It's what confirms that you aren't making things up or exaggerating. It's what tells the decision maker to believe your evidence over what the Atos assessor said, or simply to believe your evidence at all.
If claimants can walk more than just 20metres (as opposed to the previous 50metres under DLA) - about the length of two buses, even using aids like sticks - they will no longer qualify for the highest rate of the benefit. It's a cruel twist in that achieving your end goal actually leaves you with nothing.