I found this interesting video that shows a side-by-side comparison of an unnamed HD-SDI and IP Camera setup. I think the IP disadvantages are exaggerated a bit, but was still interesting to watch. In my IP setup at 15 FPS I sometimes get uneven frame rates (never the huge gaps this youtube video shows). I also notice if 2 cameras are watching the same area there is a lag between the two cameras, they don't sync perfectly. The video suggests the HD-SDI doesn't have either of these issues. At 1-8 FPS these IP issues are not noticeable.
The HD SDI / HDcctv Tutorial does a good job of explaining the differences.