i've been able to take advantage of this miles bonanza exactly ONCE.
i've found hertz prices to be prohibitive, often 40% - 60% higher than the price i get when using my best avis codes and getting 1000 - 1500 AA miles as an elite... and hertz tacks on ridiculously absurd "one way" fees when avis does not.
rent at EWR, but return at JFK? avis considers that the same city... hertz does not. rent at a casino on the las vegas strip, but return at LAS? that's a one-way rental for hertz, but carries no penalty with avis. i can sometimes find affordable rates with hertz by using other codes... but then i lose out on the UA miles. for business rentals with hertz, more often than not, i'll just use my corporate code and take the hertz points.