Obviously the D-SLR HD video is not to replace the HD camcorder, especially for action or sports video. And one should buy the D-SLR for its photo capabilities, the video is just a bonus feature and should not be the deciding feature as to which D-SLR to buy, at least for now.
However I do find a lot of use with my D-SLR HD video, especially when taking short clips of my family. The biggest advantage is its superior low light capability. I put on the 50mm f/1.4 lens, I can record in low light places where my HD camcorder will fail. Although there is no useable autofocus with video mode, I usually can manually adjust focus quickly (quicker than my Sony HD camcorder hunting for AF in low light). The image quality is excellent. The bokeh effect of large aperture lens makes the subjects in the video pop, giving a more cinematic look. Sound quality is decent, I have a Rode mic if I want better quality stereo recording. But most family clips don't need surround sound recording. It also simplifies switching photos and video with just a touch of the button (instead of fetching the camcorder from the camera bag and wait for it to start up). One big disadvantage of D-SLR video is that you need to do a lot of manual adjustments including the focusing, so it is not for those who like to leave it in full auto mode.
It is a nice bonus feature to have. I do enjoy and like it with my Canon 5D Mk II. It does not cost extra to buy the camera nowadays, since many new D-SLRs include this feature. But on the other hand, you may find a better bargain for an older model without the HD video as they get phased out. The HD video does however take up a lot of memory space on your card and computer hard drive, and you will need a better computer configuration and graphics card to play and edit the video smoothly. A QUAD core computer is usually needed for smooth operation.