Another thing the K-x lacks which may be a dealbreaker for you (but befits its low-cost status, and was the same in the K-m/K2000) is that there's no in-viewfinder indication of the autofocus points. They can still be manually selected, but are only shown on the rear-panel LCD. There's no way to see which AF point you're selecting without taking your eye away from the viewfinder, nor to see which point the auto selection chose.
Personally, I prefer custom lithium-ion batteries because they have much higher energy density than NiMH AAs. Consider for examply my 1.2 volt, 2500mAh AAs which weigh thirty grams apiece, and yield 3 watt hours. The Pentax custom li-ion weighs 75 grams, and yields 14 watt hours. That's nearly double the energy density - 5.35g/Wh for the custom vs. 10g/Wh for the AAs. To match the power output of the 75 gram custom battery you'd need 140 grams of AAs.
If you're absolutely stuck on using AA batteries, remember that the K-7's optional portrait grip will accept six AA batteries. These can either supplement the camera's custom lithium-ion, or replace it. The body will function just fine with no battery in it, and AAs in the grip. If using both types, you can also select which should be used first. It is also weather-resistant, and makes portrait-mode shooting more comfortable, since it duplicates the main controls.
It does add a bit to the size and body weight, but given that the K-7 is much smaller and a bit lighter than its main competition, it isn't unwieldy with the grip attached and is probably better balanced. For reference, the grip itself with empty AA battery tray weighs 265 grams, or 250g with the empty tray for a second custom lithium ion. With six AAs alone you'll get 18 watt hours - a bit more life than the 14 watt hour custom cell. Couple the AAs with a custom battery in-body, and you've got a whopping 32 watt hours of power on tap.
I must admit I got the grip mainly for the ability to use AAs - but as a backup in a pinch should I be unable to charge my custom battery. Otherwise, I just find AAs to be less convenient overall - the battery charger is bigger, they weigh more, and they're more fiddly to change in a rush. It's nice to know I can use them if I absolutely have to, but that's about it for me... To each his own, though.