At cruising altitude (say 35,000 feet), the difference in air pressure between the inside and outside of a comfortably pressurized aircraft is about 7 psi. If you imagine a hole just large enough for a person to squeeze through - perhaps 10 inches x 14 inches, that's 140 square inches; multiply that by 7 psi and you get 980 pounds of force - close to half a ton. If a hole that size suddenly opened up in an airplane, a lot of air would rush out very quickly and the pressure would quickly drop, but in the first few moments at least, there would surely be more than enough force to eject a person. For a small aircraft, a person would probably have to be right at the location that opened up to be "sucked out"; the pressure would fall off very rapidly with only a small volume available. For a large airliner like a 747, it could take some time for a significant amount of the air in the cabin to escape through such a hole, and there would be some pretty violent air movement near it, so an unsecured person could be ejected even from some distance away.
Two incidents (BA 5390 and Aloha 243) attest to the phenomenon, although the Aloha case was a more complex sequence of events. The investigation produced a theory that a cabin window blew out; the resulting depressurization effects caused a flight attendant to be blown into the aperture, momentarily obstructing the airflow; this produced a "fluid hammer" effect - an overpressure in excess of the structural limits of a corrosion-weakened airframe. This resulted in the "unzippering" of a large section of the upper fuselage. Despite the end result of the Aloha incident, it involved a person being effectively "sucked out" of a sudden opening in the cabin.