As a buyer I will leave feedback as soon as I receive the item and make sure it is as described. I recently purchased an item that was not as decscribed (not new, damaged). I emailed the seller stating I received the item, but it was not as described and I wanted a refund for the item and shipping both ways since the item was not what I was bidding on. The seller got back to me ASAP, we worked it out; I sent the item back, ended up with my refund and we left each other positive feedback because the transaction ended with satisfaction on both sides. I've had this situation happen to me twice in my 4 years of ebaying; both times I let the seller know that if the transaction was resolved I'd still leave positive feedback. I also leave more than "OK". I'll leave "great communication, worked out well" etc...
As a seller I will not leave feedback until I hear from the buyer that they are happy(or the time limit is up). I do not believe the transaction is complete until I know my buyer is happy with their purchase. Late last year I had a buyer email me and tell me my item was not as described, a video didn't work and she wanted a shipping refund. I knew that it worked and I knew from after-sale pre-shipping communications she had an issue with my non-inflated shipping price. I told her to send me the video back and if it didn't work in both my VCRs I'd refund her all her shipping monies but if it did work and she wanted it back she'd have to pay for re-shipping. I did not leave her feedback and I had no intentions of doing so until she left it for me. I was undecided between a nuteral or a neg for her trying to scam me but I knew (hunch) she was waiting for me to leave it so she could neg me. BTW, this was a $30 item she bought for $6 and paid $5 shipping on.