Well, enough time has passed that the D5100 is now at a sweet price point (especially the refurbished ones) that I did get myself one. There are tons of comprehensive D5100 reviews out there, this will be a short collection of very subjective thoughts from a D40 owner.
What kind of photographer am I?
Well, I'm a casual shooter. A few pics are up on flickr, but I mostly shoot family and don't really put up pictures on web galleries. My favorite subject is the human face in the middle of its many fleeting expressions.
High ISO performance
I'm very happy. Experts on sites such as dpreview complained that noise rendered D5100 photos above 1600 unusable. I was already impressed by the D40's ISO 1600 performance so I didn't pay any attention to that.
I'm extremely happy with Hi-1 (ISO 12800) and I think even ISO 25600 is better than the D40's Hi-1 (3200). This is candle light shooting, if you have an f1.8 lens. I think I'm going to have a lot of fun with the 35mm f1.8 mounted on the D5100. In addition, Nikon's auto-ISO logic will extend up to 25600 (In the D40 you would have to manually go to 3200 if you wanted that). The night shot (ISO 102400) is fun but I think the lens focuses at infinity.
Digital camera ISOs are an example of how easily we become spoilt. When I shot film the highest I ever shot was 800. I'd see 400 ISO film and salivate. Now I'm like, man, pictures aren't perfect at 25600, things could be better. WHAT MORE DO I WANT?!
I used to focus and recompose but then for some reason I got it into my head that this is inaccurate. While technically true, this only becomes an issue when shooting wide-open and medium to close distance subjects (relative to lens reach).
If you shoot wide open (say f1.8) hand held with the 35mm, trying for a subject at 1m, three things work against focus and recompose: a) The actual geometrical theory: the focal surface is actually a bowl and not flat like the sensor b) You don't actually swivel the camera in place - you shift a little. c) The subject moves during this time.
Once you stop down a little bit, or the subject is further off, the DOF is enough to compensate for focus-recompose
The D5100 has more focus points, but far fewer than the D5200. It's enough for me. I wasted enough time jogging the 4-way switch with the D40's three focus points. Sometimes, you focus-recompose and just shoot. It's better to get a little softness than miss the moment. Interestingly, cycling through the D5100's 11 points is not slower than the D40's, so I appreciate that.
The D5100 is really a step up in this department because now I have focus points above and below the midline (the D40 has three points horizontally along the midline) and this is perfect for both portrait and landscape orientation.
I am told the real utility of denser focal points is during shooting moving subjects and using AF-C, where the focus will predictively follow your subject. I'm impressed, but skeptical. Need to try this out.
I had forgotten this, but another reason I wanted the more advanced Nikons is that they had a focus rangefinder that you can use with manual focus lenses. I've tried this with my 50mm f1.8 and it works pretty well. This is a nice bonus I had not thought about when stepping up.
I find the video quality very good. My other alternative is a 8 year old Canon powershot, so my standards are modest. The video fills the screen of my laptop, and is sharp and saturated. I can't ask for more. What I was a little disappointed in at first was the focus. The hunting was a bad throwback to my compact camera days (which was a big reason why I saved up for a DSLR). However, in decent light (where our criterion for decent is now bathroom or bedroom illumination) the focus is OK. It takes about 1s for the lens (35mm f1.8) to settle down but I can see fast moving objects being a problem on a wide open lens.
This bothered me a bit. The mic, being in the body, picks up the sounds from dial adjustment, button presses and of course the focus motor on the lens. This I would definitely say, for even casual shooters: get an external mike. I was pleasantly surprised to find a speaker in the camera for playback.
Compared to m4/3
For the longest time I was looking at the m4/3 cameras, and I was looking very closely at the Olympus line, especially the E-PM-2 and the E-PL-5. High ISO, tiny size, HD video and fast AF, what's not to like?
There were two factors that led me back to Nikon DSLR: a love of shooting through the viewfinder and a lack of familiarity with the m4/3 system.
I have discovered that while I'm fine shooting with an LCD screen, I'm more comfortable using a viewfinder. I'm not sure how I would enjoy using an EVF, and that would add greatly to the cost and a little to the size of the Olympus.
I wasn't sure how quickly you could start up an E-PM-2 and shoot from standby mode. Can you leave it for ever in standby, like you can leave a DSLR?
I have a very small set of F-mount lenses which I really like, I'm not demanding at all, but I really love the 35mm f1.8 AF-S and the D40 kit lens is just fine for well lit situations where the sun is not in the front. In addition I have a 50mm f1.8 with no focus motor. It wasn't clear to me how expensive it would be for me to get similar quality lenses for the m4/3.
Lastly, while the photo experts were raving about the speed of the Olympus AF system and in principle it seemed awesome to be able to hold the camera out and then tap on the screen to focus and shoot, I just wasn't sure. If I was motivated enough I would have gone into a camera store and played with the camera for a while.
It's possible I'm passing up an opportunity to get a tiny camera system with great optics and superb video for a bulkier system with poorer video, but I'm comfortable with the price-performance point of the D5100.
UPDATE: I really liked this review at Andy's blog.