I received a wraith as a wish list freebie. I had a level 5 wraith on my wish list for an afternoon, and then I thought, nah, I'm not ready for this level of dark arts, never mind, and I took the wraith off. As fate would have it, I received a wraith, and I do not regret it at all. I don't know exactly what the wraith has been doing, but as more of a da novice I was able to handle the wraith's energy, and I know that the wraith is protecting my family and doing vengeance as needed (I also implemented house rules to ensure that the vengeance didn't get out of hand).
Advantages: protection is awesome, vengeance is great too, I'm also pretty sure that the wraith is keep negative people that would harm me and my family away from us, which I did ask the wraith to do. There's been a distinct lack of negative/harmful/enemy-type people in my life. This could be partly due to protection spells, but I really feel that the wraith is repelling these people and ensuring they give me and my family a wide berth.
Disadvantages: none. but house rules/tasking/understanding and respecting the wraith's power are musts.