I've been using the NX10 for about 2 years, and loved almost everything about it. With firmware updates it matched the later nx11 in everything. Just wish it had a tilting screen.
That, and more has been added to the nx20. The amoled screen is bigger than the earlier camera and can swing out 180 degrees and rotate 180 up and 90 degrees down. The eye-level view finder is far better than the nx10s, though I seem to be having some minor trouble with the diopter adjuster - it keeps getting knocked, which never happened on the nx10.
Handling is fantastic. At first it felt a bit awkward, but I soon got used to the slightly different control positions (caused by the larger screen.) The extra wheel around the +control is a superb addition. This makes moving through the images (even when magnified) faster, and gives you a choice of how to change setting such as the aperture in manual (either the wheel or the iFn.)
If you're taking landscape or architectural photos, the built in level will come in handy. Accessed by pressing the display button (no longer a separate button, but up on the + control) this shows both horizontal and vertical alignment. It's a bit sensitive, but still useful to have. Add in the option to have grid lines (2x2 and 3x3 plus diagonal) and converging verticals should be a thing of the past - unless that's what you want.
WiFi. For a lot of people, this may be an important selling point. If you're one of those who always post on facebook or similar, then you can post you pictures straight to one of several sites. BUT, this only works with jpgs (which makes sense.) On the other hand, while you can also email pictures, you're still limited to jpgs. I thought that maybe the WiFi would be useful for transferring images to my computer without using a card reader or usb cable (the camera is usb 2.0, my card reader usb 3.0.) In the instructions, it states that you'll have to turn off your firewall! Not a hope in hell of that happening. More irritating is the fact that once you have sent an image, you can't send it again, even if the transfer failed. So I'll ignore these features and stick to the card reader.
The final WiFi piece of the puzzle is slightly more useful, though it may become more usable with later development. You can use your android phone as an external screen and remote release for the camera. Range is approx 7 meters, but you have little control over the camera settings and worse (for me) the camera operates in "Smart" mode - the setting recommended for beginners. If you can live with the limitations, it works quite well, though there is a delay between operating the phone and the shutter firing.
Now you may think I've been a little negative on the nx20, so why the 5* rating. It's simple. The image quality. These 20mp images are really superb. I took a photo of a dragonfly with the standard lens from about 60cm. Every vein in the dragonfly's wings is visible. Like it's predecessors, the colour is excellent, being neither too vibrant nor overly subdued. Add 8fps for 8 frames - even the bracketing uses this speed where possible - and it should be quite good for sports. Maybe not motor sport though. There is a 30 frame burst mode, but that's only records 5mp images.
If like me, you use old lenses via an adapter, you'll be pleased to know that you can use manual focus assist (zoomed viewfinder) by pressing the OK button. This isn't in the manual.
Overall, this is easily a match for all but the top dslr's, but without their weight penalty. The raws are excellent and most of the features are a major step up from the earlier cameras. I personally have little use for the WiFi, and I haven't even touched the video or "creative" modes, other than taking note of them for future experiments.
If you have one of the earlier Samsungs and are looking to upgrade, you won't be disappointed. If you're coming from a compact or bridge camera, then you can make use of the beginners mode (set on by default) and know that you'll have a camera that will grow with you as your skill set expands. If you're looking for a camera that's lighter than your current dslr, then you'll probably look at that manufacturers csc range. But you really should take a look at this too. It has one of the best sensors for colour rendition, plus the Samsung lenses are a match for all but Leicas'. Give it a try. You won't regret it.