Can't comment on the K-S1, but I have the K-30 which is basically the same as the K-50, so these comments are based on the K-30. I also came from a K100D Super
1. Comfort to hold? Fits my hand perfectly. More comfortable than the K100D Super. The controls fall to hand nicely. The weight and feel are just perfect with most of my lenses. Only limitation is that it is a little unbalanced with heavy lenses like my Sigma 400mm f5.6 tele macro (weighs about 1.3kg). But you need to try one to see if it suits you.
2. Eneloops? I carried over Eneloops from my K100DS. They work fine. Haven't tested the life of them, as I have found that I prefer the Dli109s for the much lighter weight.
3. Adapter for AA batteries? I got a cheap aftermarket knockoff adapter and it works fine.
Other info?
- Nice to have video after the K100DS. But you will see in all the reviews that the video function is not great (e.g. no continuous AF, mono sound, no HDMI out). If you are serious about video, don't get a Pentax (Sony instead?).
- Some people miss the top LED screen from the K100DS, but I don't. All the info you need is in the viewfinder and on the back screen.
- The two wheels (by default front for shutter speed and rear for aperture, both customisable) are a great design. Can't understand why they dropped it on the K-S1; it's back on the K-S2.
- TAv mode is excellent. I use it the most.
- ISO is generally usable at 3200 and 6400.
- I like the RAW+jpg option. It's not a problem for me if it slows down burst mode.
- 16mp gives a lot more scope for cropping than 6mp did. File sizes are much larger (typically say 18mb for RAW and 7mb for jpg), but storage is much much cheaper than it was in 2007 when I got the K100DS.
- Button layout is fairly straightforward. Rear AF button would have been nice.
Have you considered a K-S2 instead? 20mp sensor, articulating screen, wifi, no AA filter, better AF. Smaller body and no AA battery support though.
Here's a comparison of features K-50, K-S1 and K-S2:
Pentax Cameras | Pentax K-50 vs. Pentax K-S1 vs. Pentax K-S2 - Pentax DSLR Comparison - PentaxForums.com