Hi! I have used my certificates a few times for National and Hertz rentals. They are great, because all you have to pay is the tax and surcharges. When I didn't have enough money for an entire rental, I used the certs. and just had to pay taxes. I think the last time I paid $75 in taxes for a ten day rental.
It is true that you have to call the number listed to book with an Amex cert. you don't get discounts with the cert, but it was okay with me, because it was all the less money I had to shell out for the trip. Make sure you order the certs in time. They take a while to reach you, and I had a problem with Amex not processing the transaction completely. The certs were never sent. I called from the resort, and asked that they be sent to the resort. They weren't sent again. For a $35 fee, used to be only $20, you can have them expedited. Otherwise, they take 15 to 20 *business* days.
I guess I would add that if you can get a good, really good, deal without the certs, don't use them. If you can't afford to pay for the rental without using the certs, no matter how the price might be, use them. I think when I have used them the difference in the total rental price with the certs was about $100 over the price without them. That was okay with me, because I didn't have the extra $350 for the rental. I did have points to use!
Let me know how it goes.
