Well I've had my '5100 for about a week now, and here's some things I've noticed, for anyone who may be curious.
First is the sensor really is awesome. Most of you know that already, but with proper exposure even a 1600 ISO image looks far better than it deserves to look.
I got mine as a "store kit version" with the 18-55 and 55-200 optics. I must say that other than their fragile plastic nature, they are surprisingly decent overall. The edge image quality is what I would expect from a $100 lens, but the center image quality is quite surprisingly fantastic!
The video aspect of it is OK I guess, I didn't buy it for that, but it's a nice feature to have. However it requires planning on your part to make quality video from it, it's not as simple as a point and shoot video camera. Certainly if you're in the digital cinematography realm of things it's a useful tool. You absolutely need an external mic, best off with a wireless setup, methinks.
I bought this particular body over the '7000 because I wanted the articulating screen (which is a great asset), but particularly because I wanted to use my old Nikkors with it, some of which are non-Ai Nikkors, like my 20mm f3.5 and 135 f2. I think I may have discovered something about these cameras that is unfortunate, and that is that their film plane is not calibrated for infinity focus. I think these mass-produced "low-end" cameras ('3100, '5100, '7000) rely on the DX optics to compensate, because I've yet to get a really sharp image from this camera with any of my old Nikkors at infinity. I haven't fully compared this yet to shooting a same scene with said lenses on one of my old Nikon's as well as the '5100, but that's my gut feeling about it. All of my older Nikkors look soft in comparison. Now, it may be argued that lens technology has advanced greatly, yada yada, but somehow I don't think that my (for instance) 55mm Micro-Nikkor should look worse than the 18-55mm kit lens.
On a similar subject, focusing with manual optics is very difficult with the screen in the '5100. Better luck is had going into live view, zooming in, and focusing as such. A bit extra labor, and not always practical, but in a studio or controlled environment, or any situation where you have time to compose your scene, it's not that big a deal.
Ergonomically, the camera is much better laid out than review articles like DP Review may lead you to believe. People whine about missing control surfaces, but that's just people talkin'. Like any camera as instrument, you need to learn the lay of the land, and it becomes second nature. The '5100 may not have the physical control surfaces of other cameras, but it really does not interfere all that much in day-to-day usage. This, coming from a guy who's been working with an FTN and F2SB for over 30 years. It's not a deal-breaker.
Overall it's a quite a good deal, and if you're on the fence on whether to get it or not, have no fear. Get it. It's quite an excellent machine, especially for it's price.
Hope this info has been helpful.