As I detailed in another thread last September, I had a similar experience at the JW Marriott Resort Las Vegas (switched red convertibles). Fortunately the guy who received my car also rented from Hertz and returned it about 8 hours before I did (though he didn't bother to fill the tank
) so I was only delayed about 10 minutes in order to straighten out the billing with Hertz.
I never did send a letter, but I think I will do so now as we have another stay soon at the JW, and this clearly is a recurring (but hopefully rare) problem...