Almost everyone on a long flight suffers jet lag to some degree.
A major US study by Upjohn showed 94% of longhaul travellers experience it. A survey by Conde Naste showed 93% of longhaul travellers experience it.
A 1994 survey of New Zealand based international flight attendants showed a similar result, with 96% of respondents saying they suffered from jet lag despite being accustomed to longhaul travel. Specifically 90% suffered from tiredness after arrival, 94% experienced loss of energy and motivation and 93% reported broken sleep after arrival.**
It affects passengers even more than the flight professionals. Firstly because they are generally less accustomed to the factors causing jet lag, and secondly because they are confined to a cramped space for long periods. There are also other factors such as the lack of fresh air in passenger areas.
Not all people to the same degree
Young children often seem immune. People who normally stick to a rigid daily routine, and who are bothered by changes to routine, are often the worst sufferers. People whose normal lives involve highly varied routines can often adjust their circadian rhythms better, and adapt to a disruption of normal eating and sleeping patterns. People who sleep easily can also cope better with the adjustment.
People crossing multiple time zones
The length of the flight is not the critical issue. The most important single factor is how many time zones you cross. People can suffer jet lag just crossing the United States (three hours' time change) but would be much less affected by a north-south flight of the same duration. The number of intermediate stops is also a factor, as each stop is accompanied by changes in cabin pressure. Lastly is your pre flight condition. If you are not fit, rested and healthy you will probably suffer more jet lag than others on the same flight. As a longhaul Singapore Airlines pilot says; "Everyone gets jet lag, it's a matter of personal difference as to how long you suffer after the flight."