The Nikon D5100 shoots HD video which looks a lot like the video we get from a Nikon D7000. Great for home movies, acceptable for more professional video situations, with the caveat that all the problems that plague DSLR video are still here. The jello effect is still evident if you pan the camera too fast, and aliasing can still be seen in some shots. Sound is still a weak spot: the built-in microphone on the D5100 is monaural, but it comes with a Mic. jack for an external microphone. AF is a hair faster than on other Live View DSLR cameras we've seen, but still takes time to find its focus.
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It sounds like we're knocking the video capabilities, but we're not. Shooting video on a DSLR camera gives you a cheaper alternative to get HD video with beautiful bokeh, and is a convenient all-in-one solution for both moving and still images. Still, you should know that while most consumers will be happy with the results for home video, you'll need some work to get the most out of a HD DSLR's video abilities, like getting that external microphone and a grip for steady shooting.
At the same time, shooting video with the D5100 actually offers an advantage or two over shooting video with the D7000. The D5100 shoots video at more frame-rates; 1920 x 1080 (30fps, 25fps, 24fps), 1280 x 720 (30fps, 25fps, 24fps) and 640 x 424 (30fps, 25fps), while the D7000 only hoots at the following frame-rates; 1920 x 1080 (24p), 1280 x 720 (30fps, 25fps, 24fps), 640 x 424 (24fps). The D5100 also has an articulating screen, while the D7000 doesn't - but the D7000 does have a better integrated Live View and Record lever.
Besides straight video shooting, the new effects can also be applied on video, although for an effect like Miniature the video capture will slow down and the playback will be sped up (for an inaccurate example, you'll catch about a second's worth of video after shooting for three), giving you a time-lapse like effect.
The last point we want to make about shooting video with the D5100 is that there's an odd bug or feature regarding aperture control. You can set aperture manually before going into Live View, but you cannot change aperture settings while inside Live View and shooting video. You have to exit Live View into aperture priority or manual mode to change aperture settings and then go back into Live View.