I had the K-x and K10D, and I bought the K-S2 as a replacement to both. Now, I sold the K10D but I still have the K-x. K-S2 is indeed an upgrade over the K-x on many levels - flip-out screen, better viewfinder, weather sealing, more user Control with more buttons, two e-dials instead of one, more advanced software, horizon adjustment and so on and so on... But, there is a but here.
The K-x is still smaller and lighter, and that's my main reason for keep it. It simply feels so nice in the hand. The K-S2 is larger and heavier, it do feels good in the hand and it's much more balanced when using large and heavy lenses. Still, the K-x with a tiny lens such as the DA 35 f/2.8 Macro Limited, is simply wonderful. And I'm not convinced that the K-S2 produces better images. Sure it has more Mp, so images are larger, but I wouldn't call them better. The K-x is very good. The lack of a low pass filter means images can be sharper on the K-S2 but it really depends on the lens also.