First a disclosure: I own the K-x, and only have the two humble kit zoom lenses.
I like the pictures I get from the K-x (I'm on shot count 44,661).
Done a lot of comparisons from test pictures on the web from the well known testing sites -
and of all the cameras I covet:
Pentax K-5; Nikon D7000/D5100; Pentax K-r -
I have to say I have not found enough improvement in IQ, if any - to be able to justify changing to any of them.
This is not to say any of the IQ is inferior - it is to say they are on par with each other - close enough that I would not see differences, if any, in real world conditions for my kind of photography.
I shoot under really difficult low light and strong colored lighting conditions - some ludicrously so - shooting often at ISO5000 and shutter speeds between 1/4-1/15 sec -
I have a whole thread of examples on the K-x and humble kit zooms (except a few noted "ringers" for comparison).
Kx in Use (
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Of course I'd be happy with almost any of the aforementioned cameras -
but this is the same as saying I am happy enough with the K-x that I do not need to change.