Having used the K-x for a while now I must say I love most things about it. Coming from a DS it is a huge upgrade of course. I bought an FA43 at the same time which I've used 95% of the time, so I guess that's part of my amazement of how good this camera is
Most things I really like are not common to the K-x I guess, just that cameras have evolved a lot since the DS. Things like better AF speed, better general speed when viewing photos, bigger display, more MP, SR and LV are really great! And I hardly miss the pentaprism, can't see much of a difference.
But what I really love about the K-x are the high ISO capabilities. I can bump it to 2-3000 without a problem.
Also like the output together with Lightroom 3. Don't have to PP as much as before to get good shots.