Like all others, I legitimately believed Jason Isaacs would be doing the voice of Dracula, who would have been the final boss of LoS.
However, upon discovering it was Satan, I was like, "...Ooooookaaaaay..." and didn't really expect much. But after reaching that point in the game, and hearing Isaacs' performance, I was wowed. And not to mention that it was an incredible boss fight (but nothing compared to the Silver Knight).
Overall, I think his inclusion was a plus, though I would have liked it if they gave some foreshadowing of his presence somewhere in the storyline.