Nice at first glance, but a lot of defects below the surface.
First off let me say that if you criticize a camera on these boards you will be personally attacked by an army of jerks, so its not that easy. Just take that into account when you read glowing positive reviews. Noone's paying me to speak truth about a camera.
I tested a 5100 and was not that thrilled by the camera. In fact, I returned it making sure not to get stuck with it.
The shutter is quiet and fast, and the menu is well designed, but the buttons are arranged in a unique fashion that someone else mentioned is different from most other Nikon dSLRS, so using it and another would not be a seamless experience. The live-view screen is almost more trouble than its worth, but it's mandatory on an HD vidcam. Dealing with it, however, can be a hassle for stills, since you're inclined to turn it face in, and then you have to turn it back every time you monitor etc.
The LCD is very large and pretty, but it's also quite vulnerable if you're involved in any sort of action or even travel. You have to watch the screen like a hawk. If you're shooting concerts or street where you raise your hands above the crowd, great, but is that worth the trouble the rest of the time? Maybe so, it does provide for different shooting styles, and getting low is great, too. For video, the screen is mandatory, but back later on the video...
Finally, is there a reason the camera can't come with a screen protector? You have to buy one as an accessory, and Nikon's brand name ones are no longer made. So now I have to waste time tracking down a kludge solution which may damage the screen. You pay $700 for a camera and they can't include such a critical item that's hard to find that costs 2 bucks to make? Thanks Nikon.
Beyond the great value for price, I find the default saturation setting to be really flat, so while VIVID is too much on other cameras, I find it mandatory on this one, which is odd. The shots do look great at that point. The focussing doesn't seem to be what it claims. It's good for moving objects, but average at range-finding up close, at least on my 18-135 DX lens. It doesn't hang, but it doesn't seem particularly accurate either. There's a limit to how many shots I can shoot testing, and my lenses are decent, but not top of the line, but the sharpness is not really there consistently. I've been attacked on this point. Let me just say that this camera focusses noticeably worse than the D40x. I dont care if people say its the same design. Its a much worse, less robust performer in the focussing dept, despite having more points.
The 16MP doesn't seem like much if any improvement over my D40X. Let the scoffing begin, but the JPEG processing on this one results
in about the same quality in my ultra close-up comparisons. There is more data, (fewer shots per card), but the final is about 20% better. Besides, 10-16 would be 1.6 times better. I would say 1.3 times better, and still not sure. Fine lines are blurring more at the point of pixelization.
With a few days spent on the video, I have to say it's pretty weak. The AF struggles, suffers even. You feel bad for your lenses being overworked. I know this is as advertised, but the video AF is definitely below camcorder quality, although not any worse than my Canon Vixia in low light, which is really terrible. At least dSLR lenses can be manually focussed easily depending on the lens. Nevertheless, this is no point and forget vidcam, and my Vixia beats it hands down in daylight and overcast.
What's even worse is that no matter what you set before you launch into video, it chooses the ISO for you. You can choose aperture, and you can make the camera stop adjusting exposure by locking AE, but you cannot make it stay in a low ISO in low light, and its low light video is too grainy to be used outside of personal footage. It looks great on the LCD, and on Youtube videos, but full screen, it looks much worse. I have yet to make this camera shoot low light in a low ISO, which does not show up in EXIF data like stills. The video stays the same quality no matter what kludge you try, and there are many out there, but people keep realizing that their video ends up the same no matter what.
It is NOT a low-light miracle worker in video period. Shooting video also mandates that you use battery-sucking Live-view, which stays ON until you stop) In room light or daylight, it does much better, but you still can't control the ISO. Plus if you really like shooting video, you will need the $120 in AC adapters or several batteries, because I would say the battery does about 20 minutes in video. Honestly, it even looks small, although it does charge fast, in about 60-70 minutes, which is about 1/3 faster than my EL-9.
This camera is a perfectly portable size, but otherwise a major disappointments. The D5100 is sold for a fair price but promises too much. I wanted to make the video work somehow, and workarounds are fine, and it does shoot good daylight video, but a camera that takes my settings and then ignores them while pretending it's displaying them is just going to aggravate long term. It is not the low light miracle worker its been sold as. Even stills are about a two-step improvement. I shot 1600 ISO tonight on my D40x and it looks about the same as 6400 in the D5100. That said, there's an unpleasant aspect to the 5100's grain when it shows up. It's too distinct from the background. It stands out. As I said, this sensor also has saturation and dynamic range issues.
Just thinking about how this camera is marketed as the can-do even though it's sold to a clearly consumer market bugs me. I think most amateurs won't find these problems until spending a few months with the camera, and I find the entire process dishonest. Then again, it does a lot for the price by past standards. Just don't be fooled into thinking that workarounds will make this useful to a budding videographer for instance, let alone any type of semi-pro. It's the price it is for a reason.