Interesting review. I have one of the original D90s, and it was amazing for its time and price range. The D5200 is much less expensive than the D90 was, and I guess that is the photo version of Moore's Law at work.
I too have some good Nikon glass, and while I was thinking GH3 up until I read this review, I may have to reconsider. Costco usually comes out with a super bundle on all the new Nikons.
I have the GH3 and it absolutely rocks. The footage is sharper, the audio is better, it has a headphone jack, useable real time auto-focus, built in remote commander for wireless flashes, built in wifi, it will let you do things like touch an object which the camera will then track...just awesome.
There are exactly four things that I expect to be better than the GH3 on the D5200.
1) Action sports photography with the mirror and phase detection auto-focus is just amazing on the Nikon. A contrast live view autofocus (like the GH3 has and also like the Nikon uses in live view) is nowhere near as good for fast action stills. Then there is the absence of that small amount of delay between the action and what you see in the GH3 EVF. For action stills the Nikon is better. This doesn't have to be sports either. It could be a parent following around a small child on a playground for example.
2) The Nikon 5100 (and D5200 according to reports) will be noticeably better in ultra low light. Mere low light and the GH3 will keep up nicely, but when it gets really dark, the Nikons are better than both Panasonic and Canon IMHO.
There is also the Nikon D7100 with similar specs to the D5200. The D7100 has an headphone jack but sadly a fixed LCD monitor. We shoud be getting some user feedback soon as this camera just hit the market recently.
I am debating around a GH3 or a D7100 and can still wait on a little since I don't need one right away.
Where can you find a GH3? I thought about pulling the trigger on one with everything I am reading about it. The price of the Nikon is somewhat less and available. Flip a coin or wait?
The GH3 footage looked like it was out of focus. I do like the way the D5200 renders images, but I wonder if the footage will hold together as well as the GH2 when pulling detail from black areas (