I used to love using AV (aperture value) but I didn't always get the correct exposure. After playing around with the Manual Mode it was easier to keep control of the exposure and adjust the shutter speed and aperture the way I wanted and what worked best for me.
I still use AF to focus as my eyes are not reliable for manual focus but the Focus Ring (also used when the iFN button is pressed to select settings) makes life so much easier in MF. It allows you to zoom in for a closer look and for fine tuning the focus before taking the photo. Much like using the + and - zoom buttons on DLSR's. I can tell you that having this available on the lens and not having to reach to the back of the camera is a huge time saver.
To use the Focus Ring for zooming in while focusing manually you need to have MF Assist activated by going to Menu, selecting MF Assist under the Camera Icon then selecting either Enlarge x5 or Enlarge x8.
You may also want to activate Focus Peaking (depending on what suits you best as some might find this distracting) where the camera detects the sensitivity of the focus area which displays as either white, red, green. The points that are in focus are highlighted in the colour you selected. You can increase the sensitivity but low is considered best for day to day photography where high is probably better for low light photos.
Keep an eye on the Aperture and Shutter Speed while manually focusing.
I haven't been able to find out if you can move around the screen when using MF Assist and while this is a great feature it will only zoom in on the center of the screen. To see the entire screen while manually focusing you need to deactivate MF Assist. You can't use MF Assist if your lens is set to Auto Focus, it can only be used in Manual Focus.
The following website and videos will give you a better idea of how Focus Peaking works (even though the Samsung NX300 isn't featured the idea is general). http://petapixel.com/2012/08/30/focus-peaking-making-its-way-onto-more-digital-cameras/
I may add a video later on demonstrating how this works.
Right: number of shots available, battery, Dynamic Range, photo size (20M), Drive, Flash, Metering, AF mode, Focus Area, OIS, RAW format, Fn menu
Left: Manual Mode, Autoshare, AF options, Menu, Histogram (the graph)
If the line moves to the right of the 0, the photo becomes overexposed. If it moves to the left of the 0 the photo becomes underexposed. If in the middle under 0, the photo is considered correctly exposed. This red line does not move in the other advanced modes unless you press the EV (Exposure Value, shown as a block with + and -) and manually change it.
Changing the EV in modes P, A, and S is telling the camera to make the photo a bit lighter or darker after the camera has chosen what it believes is the correct exposure for the combination of settings. Since the camera controls one of the features in P, A and S modes, you can use the EV button here but in Manual Mode you are controlling both aperture and shutter so the EV button doesn't work in the same way and exposure is adjusted according to what you decide on the settings.
Sometimes the camera isn't correct with its choice of exposure according to your settings in P, A and S so the EV button allows you to adjust for correct exposure.
For more info on EV: http://digital-photography-school.com/ev-compensation-explained
To change the aperture you need to press the EV button (the aperture will change to blue) and hold down while turning the mode dial and select the aperture that you want. Again keep an eye on the red line as it moves.
If you still struggle with getting correct exposure try adjusting your ISO, starting from 200 (if it was originally on 100) and slowly increasing until you get the desired settings. Keep in mind that the more you increase the ISO the more noise will appear but as I said before, the Samsung NX300 is excellent with handling noise.
Framing Mode is mentioned with Manual Mode in the user's manual but can be used with the other advanced modes. At first I found it confusing because when the feature is turned ON it seems to be the opposite of my Canon 650D's Exposure Compensation. But considering that the Samsung NX300 does not have an EVF, the Framing Mode seems to compensate for this. Read more about this here: http://photographyasiam.blogspot.com/2013/08/samsung-nx300-few-quick-things.html - scroll down to Framing Mode.
The following photos show the original shot and the edited shot to the right.
ISO: 100, Aperture: f/10, Shutter: 1/13 - edited with PSE 10 and Topaz Clarity
ISO: 1600, Aperture: f/9, Shutter: 1/400. I was indoors with low light so increased my ISO.
I could have decreased my aperture to increase the shutter speed but the DOF would have been limited. Here I released the ball and quickly snapped away and a fast shutter speed was needed.
There is quite a bit of noise in the second photo above but not enough to entirely degrade the photo. I could still use Topaz Labs Denoise and it would look pretty good. Shows how well the Samsung NX300 handles noise when the ISO is that high.
I did find that after extended use, about 2 hours or so straight, the camera did become hot on the battery's side but this seems normal for many SLR's. As my Canon 650D is much bigger I don't notice when the battery becomes hot. The memory card is right next to the battery on the Samsung NX300 so this can pose a slight problem as the heat may damage the card or cause it to not work correctly. If you feel the camera is getting too hot, switch it off for a few minutes and let it cool down.
In the last photo of the Merry Go Round, the bottom photo shows the Chromatic Aberration (fringing) that appears when using the 18-55mm kit lens for the Samsung NX300. In comparison to my Canon 18-55mm kit lens, the Samsung kit lens handles the fringing far better as it is less pronounced. You can reduce and mostly remove the fringing with one mouse click in Lightroom 4 under Lens Correction.
I played around with the colour presets in LR4 but the Samsung NX300 captured the colours of the Merry Go Round fairly nicely considering this was in the shade where colours tend to be muted.
Since playing with the Samsung NX300 I've started to enjoy the benefits of Manual Mode over Aperture Priority. The Samsung NX300 more than shows its ease of use, no matter what shooting mode you use.
Next: Lens Priority Mode
Edit update: regarding using manual focus while autofocus is on - this can only be done if you have a lens with the Full-Time Manual Focus (FTMF) feature like the 85mm 1.4 or 60mm 2.8 (which allows you to fine tune the autofocus manually with the focus ring without having to change to manual focus). Without this option you can/will cause damage to your lens while manually focusing with the AF on as this will fight with the camera's ability to focus.
Here's a guide for definitions of the NX lens range: Lenses explained