Ethical Hedonist and, more recently, The Photographer have both spat on me during play. It's interesting because it conveys utter contempt, is totally socially unacceptable and a little bit dirty. It carries a warning, of violence and hatred, and also the underlying messages that anything can, and will be done to you. I wouldn't say that it is especially enjoyable in and of itself as a sensation, but the context in which it is performed can be very exciting. What particularly works for me is having my mouth held open by a gag or forced open so someone can spit in my mouth, the added helplessness and intrusion adds a certain edge.
I don't feel humiliated when it's done to me, instead it adds to my feelings of being made into an object, or a Doll and these things don't get humiliated. It makes me submerge a little further I suppose, as I push down the normal feelings of revulsion and try and remain still, so it does subdue me somewhat. I think that the humiliation could come with a more reactionary roleplay - a prison scene (something that Ethical Hedonist has mentioned) or similar. Ultimately, I think it's an optional extra rather than something that will have me instantly begging for more.