For a few characters its pretty terrible. The main characters however: Victoria, Hazel, Arturo, etc. sound fucking amazing.
Some of the old characters with new voices sound close enough, but the thing I really didn't like is that some of them were not voiced at all like Nines and Bertram if I remember correctly. They should have been cut, especially as their part in the Sabbat story made no sense in the first place as Jack certainly told them not to fight over the sarcophagus and also because the dark fighting arena was one of the worst new maps anyway! Strauss could have just starved the player to Topor in his magical maze...
When I talked to burgermeister about that he said he simply couldn't find people willing to do the voices that sounded close enough to the original characters.
Although that bit is only at the end so it's not that annoying I guess.