Just returned from a week at the Los Suenos Marriott (a few miles from Jaco) for a business conference/pleasure trip. We rented a car for the week from the Hertz office in Jaco. The experience was just fine. Nice car. No problems at all. Price within a couple of dollars of the rate it was booked at through Hertz internet.
An interesting side light was that the nice man in charge of the Hertz office (Sr. Jose Solis) also books local tours of all types.
He booked six of us on a zip line tour and eleven of us on a guided tour of Manuel Antonio National Park at a MUCH lower charge than anyone else attending the conference paid through other booking agents . . . and he included lunch at the restaurant adjacent to the Hertz office for the zip line tour.
Even better, after we returned the Hertz car the afternoon before we left Costa Rica, Sr. Solis closed the office and drove us back to the Marriott. Highly recommended.