First of all, it's important to understand where the Nikon D5100 fits in Nikon's capabilities. It is considered a "high-end enthusiast" D-SLR which means that it shares the same image sensor as the high-end D7000 without some of the higher-end features. If you're like me, very few of the D7000's features justify its extra cost and weight. The D5100 offers nearly the same image quality as its bigger brother in a less-expensive, smaller package, while adding a few tricks the D7000 doesn't have including an articulating display that helps you frame hard-to-reach spots.
Compared to its lesser-priced but still excellent brother the D3100, the D5100 offers improved image quality, speed, and resolution, along with a higher-resolution articulating display. For me, this is the sweet spot in Nikon's consumer D-SLR offerings.It shoots great! Detail is way better than I expected for a "consumer" model. Actually, I chose the D5100 because the quality of the pics was my foremost goal. It comes with the same 16mp chip as the D7000 for $400 less. I wanted to spend that money on glass. I've had almost no trouble figuring out how to set the manual controls and have had good luck with the SCENES modes for most shooting. My biggest challenge has been getting correct focus as the camera tries to do way more than I'm used to and, if I have it in the wrong SCENE mode, there's no telling what might happen. The dancing yellow squares on the face recognition function is a little disconcerting. I have also had to learn patience as the VR function. I have learned to leave it off until I need it.
I think that most of the professional reviewers overstate the weaknesses of many of the products out there and the D5100Nikon is no exception, maybe because it's hard to difference a crowded competitive product line. At any rate, here is my take on some of the deficiencies. First, the location of the LiveView button work great for me. Because it is a lever rather than a button it is easy to locate and use without moving my hand on the grip. Another complaint has been the lack of an ISO button. I do prefer buttons over menus but there are too many functions which I would like to have on a button that there wouldn't be room for all of them. I programmed the fn button (which is done easily in the menus) to allow me to control the ISO. It works well both when I'm working through the viewer or the display screen. I will agree with complaints that the fn button is too close to the flash button. I've inadvertently mixed them up and it cost me a pic or two. However, I only need to make that kind of mistake a couple of times before learn to avoid the problem.
The rear screen is great...very high resolution. The zooming function works well and I have been able to check image focus easily. The side tilt is very elegant and I like that if flips to protect the screen when not in use.
I'm not much of a videographer but I did shoot some video and the image quality if fantastic. I put it on my TV and the quality was great. The focus works pretty well although you can see it occasionally get behind for a few seconds for quick moving subjects - like cars and kids. The microphone isn't worth much so it may be worth investing in the add-on microphone for more serious video projects.
All in all, the quality and fit of the camera is very good in spite of being made from plastic. i wouldn't want to drop it or the lens for that matter - they are not made like my old N90s or my 70-200 f2.8 but, on the other hand, I won't won't miss the extra 5 lbs. of weight because this camera is LIGHT! It is small but fits the hand well and is joy to carry around.
To summarize: As you would expect from a Nikon, great photos; as good as the D7000 (according to dpreview) and $400 less. Easy to use, light, good manual control, and good assisted control through scenes. The D5100 is a good choice if you are looking for high quality images without all the bells and whistles of the D7000.
No "cons" except don't drop it...probably won't survive! Battery life isn't great either (I tend to use the display and LiveView a lot) so I've already invested in a backup battery.