Definetly worth the upgrade.
Shutter - much more quiet. Faster FPS.
Build - more solid feeling. Not that much larger then K-X but little bit heavier. Second dial is great for manual or Tav. Weather resistant. LCD is usable for manual focusing with magnification,especially on tripod. There is upper LCD with important info though i still cannot get used to it and check main screen.
Sensor - 16mp, little better at high ISO, 80 lower ISO, higher dynamic range. I feel that SR is also little better.
Features - level indicator (do not use often),micro-adjusting focus per lens,interval shooting,infra remote spot on back and front of camera,much more things to adjust but less pre-set modes.
Focus - from Live View - much faster, usable. Normal focus little faster but sometimes camera says that its in focus when its clearly not.
Overall for me main pluses are build, weather resistance,quiet shutter and mainly second dial.
Very pleased with upgrade. Only dark spot is that there are dark spots on sensor
but i will send it back once rains stop and then back to my K-X with its single dial for few weeks.