The audio preamp in a DSLR is always terrible. If you're using a field recorder, either save the audio in the recorder, and synch it with the video in post, or boost the output on the recorder before sending it to the camera, and set the gain on the camera to its lowest setting. I used to use a Tascam recorder with its output set to 8 or 9, and I'd feed that audio output to my camera, which was set at minimum. Sound improved greatly.
If you're recording interviews, forget about using any kind of on camera microphone set-up. Shotgun mics mounted on the camera only have a few metres of effective range. For interviews, it's hard to beat lav mics. I use Giant Squid lavs, and they're fantastic. You could use a handheld mic, but you'd have to point it to each person speaking, and you'd have to get it close to avoid picking up background noise or reflections. Think of how two sportscasters often pass a single mic back and forth between them.
FWIW, I've stopped using a DSLR to record video, and now use a proper HD video camera instead.
