They're miles better than the Q-Tec and no-name stuff of years ago, and if you got the 500w and placed it in a basic PC which only draws about 150w then it would be fine for ages, it's just that a too few many corners are cut and if you try and pull the rated wattage it will go pop.
Ah, the old Q-Tec !
Yes you're right, if a budget PSU is only lightly loaded compared to its rating, it will probably be OK.