UNFORGIVABLE! NO NO NO NO NO!
He died a hell of a noble death, the way that Kurt should go out. They will NEVER write as emotional an ending for a character, and they will never offer a more fitting send-off. Bringing him back cheapens everything that he did and the sacrifice he made.
I love Nightcrawler and Beast more than anything in the X-Men universe, but I do not want Nightcrawler back. His death was perfect, and it pisses me off that they feel the need to do this to him.