Two caveats to what I'm about to say: 1. I'm a recent convert to White Wolf and thus don't know much about Wraith as I've really only been looking into nWoD, that being said I can't speak to anything specifically to wraith and 2. I haven't finished the book yet.
With those in mind, I can say that it seems to offer some nice unique options for RP. There is the present of a "shadow" in the form of the Geist, but I think it'll be likely that people end up playing both their Sin-Eater and their Geist. Still provides its own kinds of fun, and who doesn't like to play a PC that hears voices telling them to do the craziest stuff?
One thing that seems to be fairly unique to the nWoD at least is that most Sin-Eaters are actually pretty happy fellows. They're celebrating life and have a job to do that they enjoy.
I haven't gotten to the Krewe building chapter yet, but from what little I know about Mage: The Awakening, Geist seems to be a much more fun version of a group of magic-users. For all intents and purposes, that's what Sin-Eaters are, they just all focus on undead activities.
I'm really enjoying what I've read so far, only about 40 pages in, and the flavor's really nice so far.