I'm always a bit wary of the cheap ones, fear they are inferior...
The way I see it, 'inferior' to what? Inferior to using no filter? Personally, if there was a shot that was not possible to photograph without a certain filter, a cheap screw thread filter on hand at the time would be worth much more than a full Lee system sitting in a shop window in town.
...a bird in the hand...?
My goal in life, is to be as good a person as my dog already thinks I am.