I don't know that freight pilots are a different breed: it's an airplane, after all, and it's going from A to B, but there are a lot of different kinds of freight operations, and in general there tends to be some polarization in who flies on what. In the US, there are the top paying, top performers such as UPS and Fedex. There are smaller operations that feed the big boys, carry the mail, etc, and some airlines have their own aircraft that do only cargo (Northwest, for example, had it's own cargo operation). There are bush operations such as Everts in Alaska, that flies (among other stuff) classic aircraft like the DC-6 and C-46. Then there are the supplemental operations.
A few years ago the rest and duty regulations were re-written in the US to address fatigue more fully, and for the first time incorporated circadian issues (jet lag). These changes affected all the airlines, except the supplemental cargo, who were completely left out. It's sometimes referred to as the "great carve-out." That may tell you something about flying the supplemental side.
The supplementals (eg, Atlas, Kalitta, Southern, Western Global, etc) tend to fly for a number of different clients as well as carrying their own contracted freight, and often do a lot of military flights. It's common when there's a disaster to see a lot of use of supplementals to respond to disease outbreaks, earthquakes, tusunamis, etc with relief supplies. It's supplementals that you'll see doing most of the heavy lifting in and out of places like Afghanistan.
The type of cargo flying one is doing, the type of equipment, and the client or employer (or both) make for widely varying situations for "freight dogs." There's a popular logo seen around that shows a dog smoking a cigarette and says "OOSK" which is "order of the sleepless knights." Flying freight, one may end up on both sides of the clock on a regular basis, all over the world.
A groundschool instructor at Kalitta used to like to say "we consider ourselves the cream of the crap," and also "we stay next door to some of the finest hotels on earth."
More of the older aircraft are gradually winnowing out, but even the "new" equipment is often old. I can remember referring to the B747-400 as the "gucci bird" because it was so new and exotic compared to our Classic 747, while most of the world looked at the -400 as tired, old, and antiquated. It's relative.
I lost track of the number of rocket attacks I sat out, on board, in Afghanistan, while we had no way off the aircraft as no one would bring stairs.
Tech stops in Khabavorosk, watching the runway deiced with a jet engine on a truck. Paper plotting charts (going away soon). Scratchy HF radios. Getting on board the aircraft to find it loaded one end to the other with pallets stacked two-high of Lithium batteries...and each one placarded with large labels that said "do not stack." Airplane loads of cows to Kazakhstan. A killer whale in a tank. An airplane main deck lined one end to the other with missiles and explosives. Standing at attention during a solemn dignified transfer of remains as a serviceman is returned home, having carried him all the way from where he fell, and watching as his family receives the casket. Incredibly long legs. Rest facilities occupied by a mechanic who hasn't been off the airplane in 30 days. Cans of coke and bottled water floating in a filthy cooler that's probably got diseases floating inside that haven't yet been classified by science. Dysentery at night in the middle of nowhere with nothing to do but run aft every few minutes. Kidney stones in Abuja, Nigeria. Bodies in the wheelwell, frozen, stow-aways who would rather die than remain where they are. Flights into Ebola areas. Flights that change callsigns several times during the flight, while still airborne. Sitting in a ground school with a group of airmen who have, collectively, experienced every abnormal and emergency procedure in the flight manual, and a few that aren't. And on and on.
Many of the supplementals are filled with very experienced aviators who lack a degree and can't apply at better paying jobs. There's no lack of skill or experience, but for whatever reason, many find themselves at the freight operations not entirely by choice, or because it's comfortable, or close to home, or whatever their personal reason may be.
Some freight aircraft carry a mechanic on board, parts, and a loadmaster. Some freight operations require the pilot to do the loading and off loading. In some operations, the pilot ends up filing his own flight plans and doing his own flight planning. For many of the globe-trotting supplementals, the operations are conducted and feel more like a "Part 135" charter operation, just longer legs.
There are some freight pilots who want to get their days on the road done and get home. Others see it as an opportunity to see the world. I knew one pilot who loved to photograph; he'd go out at each layover to go into mosques to get images of colorful prayer rugs rolled and stacked, or old boats by the water. He'd promise those who let him take their photograph, that he'd bring prints, and sometimes he went back month after month searching until he found them and gave them their picture. Some simply go from bar to bar, brothel to brothel. Others do their college education while they're on the road, or study a language, or write.
There are few jobs that allow the variety and diversity that many cargo jobs do; some are out and back between the same two or three airports, and some involve several legs a night in a worn out Beech 99. Most involve a lot of night flight. It's possible to go all month and never see the same airport twice. Sometimes for months at a time. It's hard to beat the thai curry in Entebbe followed by a breakfast in Liege, a quick turn with a green lunch in new york and then something local in Sao Paulo...everyone marks their trips and their work differently.
Everyone should have a chance to visit Louisville or Memphis and see how the cargo is handled; it's quite a wonder. Incredible, really, to see the whole machine at work. Impressive. At the same time, to stand at the head of the stairs in Karachi and be beckoned by a short dark man holding an MP5 submachine gun, to learn that all he wants is a pepsi...you meet interesting people in interesting places that may be full of wealth, poverty, or war, often a mix of all, and whether it's in this country or that, it could be said that the freight dogs are a unique subset, or just that it's a group of tired crew having unique experiences in interesting places. Cool aircraft, cool guys to fly with, some great food out there, and personally, far more than I ever thought I'd see in a lifetime.