Personally I feel the whole feedback system is way overrated. I leave feedback whenever I get around to it. Plain and simple, wether I am the buyer or seller. Sometimes it takes me a month, sometimes more, sometimes less.
People are so afraid of getting a negative that many seller and buyers do not leave negatives even for those who deserve it, so you can never get an honest view of what an eBayer is like.
I don't think many sellers even look at a buyers feeback history. When I was rated about 40 I recieved 30+ retalitory negatives from a seller that was a PITA. (a term I learned on the eBay boards.. Since then, I have yet to have another seller cancel my bid because of my feedback history. I just hit 100 the other day