It took me a little bit to get warmed up to the concept, but now I definitely see the potential for using DSLRs for movie making. Camcorders (in the price range I would consider) are fitted with single lenses (probably a superzoom) with average optical quality. Their smaller sensor size means a much noisier low light performance.
With this cheap DSLR I can put on my cheap 50mm/1.8 and get HD movies that look 'arty' because I opened the lens up wide. I can take movies in indoor lighting. I can take videos of my cat that look like something showing at Sundance. It really opens up for creativity.
Ok, you know how guys get a bad rap because they don't read the manual? I'm one of those guys. This ain't your grandpapy's SLR. You flick the switch to live view. You set to AF-C and select 'Tracking' for mode. You use the eight-way switch to place the green cursor over your subject. Then you hit OK. This registers the pattern in the targeting computer and the square will follow the subject around the screen and keep focus on it. I think this is what fighter pilots use to target their AGMs.
In the meanwhile, manual focus works a blast, and I can see this working just fine stopped down to f3.4 or so for causal home movies.
I read somewhere that movie makers actually use manual focus. I remember seeing documentaries about film making and the camera-man's assistant was always scuttling around with a tape measure and the actors had lines on the floor indicating where they were to stand for the low DoF shots, so perhaps no AF logic can really understand what YOU want in focus in a moving scene and so you need to do it manually.
Normal battery life seems to be fine. I say seems because I have been using the video a bit and, man, does this chew through the battery fast. I had a fully charged battery and I had it down to the last bar within 30min of playing with video, making about 4 videos in this time and having live view on. I suspect that if you are using live view a bunch/making videos you need to find a very high capacity battery or have a bunch of spares. On the days I went out and shot mostly stills, I did not have any problems, the battery went through 100 shots without changing a bar.
I've been using a cheap(er) Wasabi battery and it has worked fine so far. Some people complained that their D5100 did not recognize the battery, or did not recognize it after the first charge, but so far, so good.
The view finder now carries complete exposure information. You see shutter speed, f-number AND ISO in the viewfinder. This is also customizable, which makes it a nice step up from the D40's viewfinder. For some reason, I find it easier to manually focus my 50mm/1.8 on the D5100. I suspect that it is the rangefinder display that is aiding me, though the added brightness may be helping.
My use of the viewfinder is primarily to check focus and secondarily to compose. I'm OK with a little composition slop because I feel I can crop later (I don't worry too much about sticking to standard aspects). So if the coverage (how much of the scene is visible in the viewfinder) is less than 100% I don't care - I'm possibly getting some junk on the edges which I will crop away if needed. But I worry about focus and this matters a lot in low-light. So my intuition was that viewfinders with more magnification are better to see details which are important for focus.
Then I went through some specs on film and digital cameras:
F65 (Film camera, entry level) - 0.68x, 89% (Pg 106 of manual)
F4 (Film, top end, interchangeable viewfinders) - 0.7x, 100% (stock), 6x (special) (Pg 107 of manual)
D40 (Digital, entry) - 0.80x, 95%
D5100 (Digital, entry) - 0.78x, 95%
D4 (Digital, top end)- 0.70x, 100%
That's odd. Why would the top camera have LESS magnification than the entry level ones?
I read a nice description of basic viewfinder specs on luminous landscape and on stack exchange photography. I always thought that a larger magnification was better, but Stan Roger's answer on stack exchange made me realize that as the magnification gets larger the image gets dimmer, so there is a tradeoff between detail and low light visibility (other trade-offs are mentioned on the luminous landscape article).
This is a double hit: it is in low-light, when your f-number is low that accurate focus is most important (and your AF is likely to give up) but your viewfinder has to tradeoff between being visible and giving you detail.
So, I guess, in the D4 and other FF cameras, the designers went for better visibility (users often describe the viewfinders as brighter) deciding that that would help manual focus with 0.7x mag giving enough acuity for focus.
As a sidenote - where are our interchangeable viewfinders for digital cameras? I don't think even the D4 "flagship" has the ability to pop-off the stock pentaprism assembly and place another viewfinder.