Actually, both of the requests here are already part of FlyerTalk.

We already have car forums as noted by this forum which is titled "Car Rentals Programs/Partners":
http://www.flyertalk.com/forum/forumdisplay.php?f=392

As for the other request about sharing experiences at the vehicle level, well, we also have that and please feel free to add your reports. These are called "Ride Reports" on FlyerTalk and here's the current listing from the Hertz forum:
http://www.flyertalk.com/forum/showthread.php?t=426432

Members can feel free to add a Ride Report at any time and then we'll try and see that they are cataloged in a sticky as is done in the Hertz forum.

Ride Reports are much like a Trip Report involving a certain model car. You'll also see that we encourage Ride Reports for cars outside the U.S.

Hope this helps.