I've been very happy with all my car USA car rentals when using AutoSlash - they have certainly saved me money.
However, do not use them for Mexico car rentals. They don't include the Mexican mandatory liability insurance in their price quote and it's very deceiving -- they hide a disclaimer in very small print -- and the result is that you show up in Mexico expecting to pay a low price, and you are forced to pay 5X that amount instead. Other sites like SIXT are much more transparent and give you the correct and complete quote, so use SIXT for Mexico.