I have used autoslash at least a dozen times and have saved money almost every time. While the initial price quote may not seem much different that other websites, if you register your reservation, they let you know if a lower price becomes available for your dates (taking into account any discounts you are eligible for like AAA, etc.). If you choose to accept a lower rate, you do have to book at the lower rate and cancel the initial reservation. I always then followup with a phone call to the rental company to confirm the reservation and be sure my frequent renter number is added. I just saved $100 on a 3 day rental in Alaska because of autoslash. I've never had any difficulty picking up my car or being charged more than quoted. I love this website.
Ps you can register a reservation that you made yourself on another site or a reservation that you made through autoslash.