Just to add to what Adam said, I used to be a die hard fan of AA batteries too and indeed I had a K-x myself (great little camera) but was very surprised how I changed my tune when I got a K-5 (with a spare battery). I really appreciated the improved battery life (never had a problem in the field), the compact battery size (light and easy to keep spares handy) and even the charger is fairly compact. I was intending to get a grip so I could use AA batteries with the K-5 but I never bothered!
As it turned out our family now also has a K-01 and a K-1 - which all share the same battery type (increasing potential spares).
If you are like me then I probably can't convince you change from AAs, but from my experience moving to Lithium batteries have been almost entirely positive experience. Also I agree with boriscleto that K-r is not worth upgrading from a K-x.
Edit: forgot to add that there are after market alternatives to Pentax batteries too which are much cheaper - but be sure to pick one from a reputable manufacture. My K-5 was second hand so I was not too worried about using after market batteries and have had perfect problem free run. I don't use them in the K-1 (while its in warranty anyway) but thats just me being cautious with such a lovely piece of kit.
Last edited by kiwi_jono; 06-07-2017 at 10:51 PM.