I personally wouldn't have a problem with Hayden being a ghost, if they'd reshot the unmasking/death scene with him too (and aged him accordingly and given him some acting talent). It would at least have added some much needed continuity that way.

Part of my problem with him replacing Shaw is due to his awful performance in the prequels. If he'd done a good job, and we saw him as the kind spirited Jedi we'd always assumed Anakin to be, seeing him in ghost form would not only be forgivable, but an improvement on what we currently have - someone who is alien to those who have watched the films in chronological order.

That child molester grin Hayden has in the final shot doesn't help either. What a way to end a saga.