After 7+ years of using a k100d, I picked up a k5iis a little over a month ago. It's a supremely polished machine by comparison, I'm sure you'll enjoy this new generation.
-Read the manual cover to cover. It's packed with customization options. It can now remember the drive mode you were in even after you powered it off. Or not. Your choice. So much more customisation then the k100d
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-The bigger, brighter viewfinder is the bomb, your incoming M50/1.7 will love it.
-The speed is just unreal, especially if you're used to shooting raw with a k100d. The buffer space on a k5iis seems close enough to infinite to me
, and 7fps feels like an out of control rocketship.
-Autofocus is now use able indoors (hooray)! YMMV, but it's a massive upgrade.
-The auto dust removal feature seems excellent so far.
-That front control wheel, while awesome, is still getting accidentally bumped by my fingers whose muscle memory expects nothing there.
-If you're a pixel peeper who likes to view things at 100%, keep reminding yourself that the 16mp image is being subjected to more magnification then you're used to.
-I'm also using the two old kit lenses as well as a FA50/1.4 and a DFA100/2.8. Compared to the latter two, the kit lenses felt sad on the k100d. The extra resolution on the k5ii makes the kit lenses feel relatively sadder in my opinion, so don't expect much more detail out of them. However, the k5ii will feel more capable of 'getting the shot' in many instances, or at least making it easier to get (better high iso performance will definitely help). Fast, quality glass is still king, but there's no reason your k5ii shouldn't last you another 7+ years, so there's time to add to your lens collection
For your k100ds, keep it as a back up. Get eneloops as mentioned, preferably with a smart charger. The high capacity XX eneloops are even better if you can find them for a decent price. With the k100d, I am in the camp of not fearing iso 800, venturing into iso 1600 when needed, but only iso 3200 in dire emergencies. I've yet to sort out my iso thresholds on the k5iis, but even at an unfair pixel level comparison it feels at least a stop better, maybe two. In any case, as the iso goes up, hitting the exposure dead on gets more and more important. You also might try your flash in manual mode, it will take more work setting up but not having the p-ttl flashes firing away can make it easier on the subjects.