But, I really can't leave feedback until the purchase cycle is complete .... can I?  Until I receive the things I've bought, how can I leave honest feedback?

of-course, you CANNOT leave FB until you are absolutely sure it is what was said in the seller's description, and it is working properly; that is why eBay gives you 60 days to do it.  Has any of the sellers been asking you to leave FB before your item arrived?  If so, please give us the exact title of the item (NO numbers or seller's ID)