Of the Amex Platinum Card benefits, I find the Avis ones the least worthwhile. If you don't otherwise have National Executive status, I agree that it is a great benefit, although I've been disappointed in the car selection. I also really like the 4-hour cushion at Hertz, though I often find better rates using the AAA code--how much value it offers really depends on your plans.
I've found National to be very accommodating, but I rent from Hertz more frequently because they seem to have better rates for my most frequent rentals--I've never managed to find very good National discounts for my typical routes.