NIKON 5100 for video???!!!
I got my D5100 in January and use it for a mix of photos & video.
Overall I am quite happy with the results from the video- though some shots when you pan do produce quite a bit of flicker- not sure if this is an effect from the rolling shutter or not.
I use A mode and set the aperture before I go into Live View. If you change the aperture once in live view you have to either exit live view and re-enter or take a photo while in live view for the changed aperture to be used! You cannot as you say set either the shutter speed or ISO- nor can you actually tell which ones the camera has chosen as the ones displayed on the screen only apply to photos not video. However you can alter the exposure by + - using exposure compensation. It is also a good idea to lock the exposure using the AE-L button before filming. I use manual focus as much as possible, though like all LCD screens it can be tricky in strong lighting conditions and I am now considering getting a loupe to add to the back of the camera. Overall I am very happy though it does require quite a bit of effect on your part to get the best results- but thats part of the fun!