Agree with Monsterkitty. We're heading to Oahu in a week and started looking at rental cars through different sites. Costco by far is the easiest and almost always had the best prices - and that 2nd driver is a good perk for us too. We started booking a car back in Sept and kept checking every week or so rebooking and canceling along the way. Once we got to the beginning of Dec we started checking a little more frequently.
The rates started out at about $1500-1700 for an 11 day rental (checking all categories, mostly compact or economy), over about 6 changes we're now at $640 for the 11 days in an intermediate (the compact was $12 cheaper, but we elected for more luggage/leg room). Not as good as we've gotten before, but I don't think it's too bad.