local HLEs usually give upgrades in my experience since they to some extent get to control their inventory and try to only have nicer cars availible. at the airports, my experience has been mixed (it's been better recently) - sometimes I get a nice upgrade (compact to Navigator), but usually I have to ask for it by going to the gold desk. If it's a shorter rental, I have a better chance to getting the upgrade.

strange thing is I never get the upgrade when I want it. Like if I'm going on vacation for a week nothing, I get stuck with the car I ordered. If i'm in rainy Portland at a conference where I'll only be driving from hotel to hotel, they give me a convertable instead of the compact I rented to save the company a few bucks.