Several people are talking about field vehicles and I guess I'm lucky but I've never had to rent a field vehicle. (I do have a story about that for later.) I work for the State and we have our own vehicles, mostly trucks and SUVs, which while low mpg, they do meet all the general field vehicle requirements. (Can I get a pencil/map/GPS/field book holder, too?)
Picking up core requires (sometimes extensive) driving on bad lease roads and carrying several hundred feet of boxed, 2-inch core in the bed - this limits our choices to 4WD full-sized trucks; even then some of the hills were iffy, also you don't jack-rabbit off from a stop. Luckily the truck that could haul the most also had an extended cab because all three of us rode to lunch in one truck. (We're State employees, we're taking our lunch break!) For looking at actual outcrops, we use one of the SUVs - space for people in relative comfort, room for samples, good clearance so we don't have to worry about what's on the shoulder, etc. The mappers tend to be pickier and a little possessive about the trucks. (Big Blue is Ed's truck, the red one with the ugly camper shell was Dorothy's...)
Most of the field work I've done since working here was at our "old test site", a short way down a decent lease road, where anything except the mini-van or Crown Vic would do. (Actually, no one really wants to drive the Crown Vic or mini-van on the highway either. We also have the "mail van" that's rather scary but it still makes to the PO.) Our new test site (where we might actually get to put some CO2 in the ground!) is really close to the highway and on a great lease road - I could drive my brother's Mustang up there, but we haven't brought in drilling and work-over rigs yet. The agency's vehicles get well used, a lot. High mileage is the rule, but for the most part they are well cared for but where I worked right after college we had a couple of real doozies.
The worst vehicle I think I have ever driven was a Dogde dually with possessed alignment. This truck would pull to one side long enough for you to start to get used to it and then it would switch, suddenly, to pulling to the other side. Or the amount/strength of the pull would change. It was exhausting to drive. One time couple of the guys were driving back to the hotel after doing a tank tightness test (well after midnight) and were pulled over for weaving. They hadn't had a drop of alcohol, they were just to tired to fight the steering wheel. There was also an old Explorer that'd shake your fillings out above 50 miles/hour; that one was loads of fun to ride in to Montgomery. At that job there was a definite hierarchy of vehicle allocation: owners - new(every few years) SUV (for work and personal use); engineers going to meeting - newish SUV/truck; survey crews - slightly older SUV/newish trucks; environmental division - a couple of trucks that wouldn't be replaced until they wouldn't tow the grout machine; geologic division (both of us) - whatever was left or POV. A couple times I was very thankful for my station wagon had front-wheel drive.
The University's vans were another set interesting field vehicles. Apparently years ago my Alma mater rented the vans in New Mexico and everybody just met in Albuquerque for field camp. Then the rental companies figured out what they were doing with the vans and stopped giving them the corporate rate. After that they started using the university's vans but the motor-pool people had already figured out not to give the Geology department nice vans because we'd get them all muddy. When I was at field camp both vans had a crack in the windshield, top to bottom. The whole set of vans purchased one year had the windshields badly seated and they all had cracks. I can assure you there was no way you could rev the engine up enough to warp the frame (like in drag racing); seriously, a semi passed us one day, going up hill - can you say under-powered? (Still better than under-braked.) On a southern Appalachian field trip we had a pair of under braked vans - wouldn't erring on the side of over-powered brakes be preferable to possibly under-braked? - We only had to take one brake-cooling break, thanks good route planning and 1st gear.
The only rental car/ field experience I had was near Denver. My advisor and I were there for a class and had a free day, so we went out driving. "Unimproved road" means something different in Colorado than it means in Alabama. In AL it, generally, means county maintained dirt; I think in CO means any track one could conceivably drive some sort of land vehicle over. We started on a paved road which became dirt which became a jeep track which started to look a lot like the photos Silver Fox posted. We decided after a short distance that perhaps we didn't want to have to walk back to civilization and turned around. My advisor told me a story about a field trip where they'd reserved high-clearance 4-wheel drive vehicles and were given sedans. IIRC, he told me they returned 3 and drew a map to the 4th.