I pretty much only rent from Hertz, use Gold service exclusively, but here is the honest truth about it.
When it works, it's a great service just to see your name on the board, go to the parking space noted and go (contract is already printed, just show your contract and ID at the gate).
That said, there are many times that I have not had my name on the board, whether there was a flight delay or not. This means, off to the Gold Service desk to have a vehicle assigned me. In not Gold Canopy locations, I've had the best luck with the contract already being printed and keys set aside with usually an upgraded car, while in the Gold Canopy locations, usually I get what I reserve.
I typically reserve vehicles from the Prestige and Fun collections, so the experience I have typically revolves around rentals outside of these two collections. From experience they tend to be more atuned to specialty rentals then run of the mill rentals.
If you're a new Gold Customer, I wouldn't expect an upgrade, you'll probably get the class you reserved and don't expect it to be really anything different from the regular fleet selection. Even President's Circle (PC) and Five Star (5*) members don't always get upgrades, even though Hertz touts this as one of the benefits.