I have a Truper brand #5 plane, basically a crappy copy of a Stanley. Mexican brand but probably made in China - you know the drill.
I've fettled the plane enough to the degree at which I think is appropriate for a jack plane.
The iron is not stellar, but it works. However, the chipbreaker has always been terrible. It's too thin and soft. It never matched the iron well, even though I filed/polished it carefully whenever it got out of whack. Bits of shavings always managed to get themselves between the iron and the breaker.
I got a Veritas cap iron and put it to work two weeks ago. The plane has never cut better. It doesn't chatter any more, and shavings don't get stuck. I guess the rigidity of the Veritas (I can't bend it with my hands, whereas I could with the original one), plus the fact that it is flat and matches the iron well, are what make it work so nicely.
Summary: crappy plane, well-fettled, with non-special iron, but with good breaker = pretty damn good.