I rent cars 4 to 5 months each fall/winter. Last year AutoSlash saved me $5-600. This coming year they have already saved me over $400 in bookings. I tend to find my best deal on Kayak then list it with AS. If and when they find a better rate I will book it then list that reservation on AS.
AS is free and saves me searching to try and find newer and lower rates. 90% of my rentals end up AS sourced.