I seldom recommend using a third party for any type of reservation....too much can go wrong. I usually just go to the official sites. Each car rental company offers their own specials at various times...so you do have to keep checking and comparing. It's not as easy as using a priceline or expedia where you can compare costs in one glance, but it does seem to be less expensive.
We have used car rental savers twice now. No problems at all-you can go directly to the website of the place you are renting from (Dollar,Budget, etc) after you reserve and check your confirmation there as well. I like how it compares all the companies side by side- it takes so long to go to each site individually and put in different coupons.
http://www.autoslash.com/ This is one I have been checking that was recommended on here by someone. It is supposed to keep checking for you and rebook at the lower price.