So feedback left for a buyer is information for future sellers (and to a lesser extent buyers) that deal with that eBay member.
To leave feedback that only states that the buyer has met their payment obligation is to leave incomplete information for those people.
If you have respect for the wider eBaying community, you will do the right thing and wait until the end of the transaction (that is, when all is known about the transaction), regardless of whether you end up leaving feedback first or not.
The following added 24 October 2007:
Feedback for Buyers
I truly believe that there is no place for the Neutral in buyers' feedback for sellers, especially now that eBay virtually treats them as negative in terms of Seller Performance.
So, as a buyer I would recommend that you do the following regardless of whether the seller has yet left feedback.
If you are basically happy with what you've received, the state it is in and how much you've paid for shipping then go ahead and leave positive feedback. The seller shipped your item, so they know you've paid. There's no reason for them not to leave you positive feedback in return (if they haven't already).
If you feel that there is a problem with anything, please make every effort to contact the seller first. Yes, they should be reasonable in handling your concerns. They should answer your emails in a timely manner. Depending on the value of the item a phone call might be merited. Only when all avenues of negotiation have been exhausted should you then leave negative feedback. The leaving of this negative feedback indicates that you are no longer willing to negotiate in a conventional manner. Do not be afraid of the feedback you may receive in return. It might be hard to accept an "undeserved" negative feedback, but if you've done the right thing by yourself and your seller in negotiation then you can wear it with pride. Make sure that you purchase with a buying ID (have a separate eBay ID for selling) so that you can do this without fear. Do not negotiate further based solely on feedback left for you. The only reason that you should consider removing this negative feedback is if your original concerns are handled in a satisfactory manner; you may or may not consider tempering your original demands at this phase, but resist the temptation to factor in any bad feedback left for you in any further negotiations.If the item is of little value and the eBay Community as a whole is not "endangered" by the sellers practices, then consider leaving no feedback at all rather than negative. An example of an item of little value that might "endanger" the Community would be counterfeit merchandise, such as a movie DVD. You might also consider a negative on an item of little value if the seller was particularly negligent or abusive in their dealings.
Next: Feedback for Sellers
Most importantly - have fun!
This is a work in progress. Due to the new feedback policy being introduced by eBay, this feedback guide is to undergo major revision. Please check back soon for that revision.