Most rental companies have either "non-smoking" cars or no smoking in any of their cars. For those of us who have allergies or are sensitive to smoke, being in a car that someone has smoked in is torture.
I've had nothing but good experiences renting cars. I'm 25 years old and have been renting them a few times a year since 2007. If you're in the military they will lower the over 25 rule to over 21. I've only had the "we ran out of the car you reserved" problem happen twice, and both times they upgraded me to the next size vehicle for no extra charge.
And if you're wondering why the car you got smells like an ash-tray, that's because I smoked in it.
"We don't care what you reserved." So true! One time, we arrived after midnight. Even though we had a reservation, the guy had to retype all of are info and it took FOREVER. Then, we went out in the lot and there were only three cars left, all of them "mid-size" (we reserved a compact). We picked one and then they stopped us at the gate, because we had a "mid-size" instead of a compact. We turned around and drove back to the office and had those idiots call the idiots at the gate and we finally got to drive to our destination! I think that shipping my bicycle to my destination would be less hassle!