Well, I was a happy K-m owner for ~1.5 year and upgraded to K-5 last year (I was one of the first adopters in my country). I loved the K-m and I still think it was a nice and capable camera. But the K-5 offered and offers very nice improvements for me in
- dynamic range - no more bracketing, just meter for highlights and can pull the shadows later without worrying about noise / IQ degradation
- high ISO performance (I shoot a lot in low light where flash isn't always an option)
- AF - AF-S seems to be noticeably faster and much less hesitant in locking, AF-C is miles better
- handling - a subjective thing, but K-5 fits my hand better, I'm about to order a grip to make portrait shooting even more comfortable; more external controls -> less menu diving; USER modes -> very handy for me
- better rear LCD - not a big deal, but a nice improvement for checking photos
I thought K-5 would be too much for hobby snapshooter like me and wouldn't be able to justify purchasing it, but I knew I would lust for it. I'm glad I made the decision to purchase K-5.