I've had very little neg rep - 3 instances I think - and handed out 2. Once a person mentioned deciding "not to neg rep back" - but that is against the rules of the rep system in any case - and I am sure the Admins would act to remove such an obvious breach if brought to their attention.
When I got an unsigned neg rep that said "you are a piece of ****" I was really taken aback as this community is usually very intelligent and constructive. I actually know who it was and I sure wont help the guy out again in a hurry - I had offered a lot of help and then did one thing he didnt like.
I lost 1 rep point. boo hoo. Had he made an intelligent comment and signed it I would feel differently - but cowardly insulting behaviour such as that shows a persons true colour in my experience. He has lost the benefit of my experience, knowledge, patience and generosity - I doubt he has the interpersonal skills to ever win them back.