I'm not sure what the attacker experiences, however, a friend of mine is a psychic vampire and she'll occasionally feed from me. I can feel when she first starts and if feels like a pulling sensation. Not like a child pulling at your pant legs, or someone dragging you, but a pulling sensation, I can't really explain it. After that I just get very tired, and slowly shut down. Then I'll sleep until I've replenished what she's taken. I've only been fed off of by 2 people, and both had permission.
As I've stated before I'm not sure what the attacker feels but I am sure it is something like a re-energizing feel, or a refreshed feeling, or something similar.