I rent cars and trucks quite a bit. I usually reserve a compact or mid-size. I have been upgraded about 40% of the time. A few that I can remember>
In Vegas, I got up'd to an Audi A4. Great car.
In PHX, I got up'd to a Cadillac SVX suv.
In San Diego, I got up'd to a Camero convertible. 33 mpg to PHX and back.
In San Diego, I got up'd to a Pontiac Gran Prix GT and ended up buying one
In SLC, I got up'd to a Town & Country mini-van. Those new vans are nice.
I have found that 2-doors and convertibles are not as popular and can be had by request at the counter with no additional charges.