For my 9+ hour journey to Boston from D.C., I figured that I needed a rental truck to haul all my stuff from the condo to the new apartment - I definitely can't fit all of my belongings into a Prius! Now that I've had this experience with rental truck companies, I think I have some helpful advice for others who may be considering this option for a move.
1) Look up different companies online to find out their best prices -- they will vary company to company. Since I was doing an inter-city move, the options were fewer than if you are just moving within one city. Most companies won't rent you a moving van in lieu of a truck if you are returning it to a different location.
2) Make the reservation a couple of weeks in advance if possible -- there'll probably be fine print when you make the reservation saying that you can only change or cancel it up to a day or two in advance. Here's what I learned that can really save you money: After you've made the reservation, call back and say you want to cancel to use another company. They want your business, and they don't want to give it to a competitor, so they will reduce your price. I had a 12' Penske truck reserved for $450 (the most competitive online price), and called to tell them to cancel because I found a better offer on a Uhaul van. They reduced the price to $250 -- a huge difference! If I hadn't done this, I never would have known that you can haggle with these rental truck companies.
3) Be prepared to pay dearly for some gas. Ouch. We made it all the way from D.C. to Connecticut on one tank of gas, but I paid $175 to fill it back up (gas was ~ $4/gallon at the time).
4) When you make a reservation for a certain-sized truck, you may in fact receive a larger one. My reservation for a 12' ended up with 16' truck -- way more than I needed! But when I expressed dissatisfaction to the manager, he threw in some extra tape and other supplies, which was nice. And supposedly, the 16' gets the same gas mileage as the 12'.
5) I never realized before just how many roads there are that do not allow trucks. These rental trucks qualify as real trucks, so you have to stop at the weigh stations and can't go on roads that say "no trucks". I also had never noticed the road signs before limiting trucks to only certain lanes of the highway.