There are two ways of looking at IS. One way: IS is an expensive way to (sometimes) get away with bad habits in your basic photographic technique. The other way: It is a tool to extend the range of very good photographers who do use good fundamentals.
For the cameras discussed here, I tend to see it mostly as the first viewpoint. There are a few exceptions. If you take a lot of pictures from moving/vibrating platforms such as vehicles, it can be helpful.
In the hands of amateurs, IS can allow shots up to 3 stops slower for the same focal length. Because pro's get much closer to the theoretical limits of their equipment to begin with, adding IS usually only extends their range about 1 or 1.5 stops. Sometimes though, that extra range is the difference between getting or not getting that BIG money shot, thus worth the expense. But we are talking VERY expensive D-SLR equipment here. In addition, under such scenarios IS is being used in conjuction WITH (not INSTEAD of) other ways of stabilizing the platform.
One of the most familiar applications to a lot of people would be the sports photographer. We all see them on the sidelines at the NFL/NBA/etc... games every weekend. There is no more severe test of photographers and equipment than shooting fast moving subjects (athletes, wildlife, whatever...) in low light at high focal lengths from a distance. They are using very fast lenses (a "fast" lens is one that lets more light through to the focal plane so that a faster shutter speed will allow proper exposure) at high ISO settings (requires less light to begin with, but the tradeoff is digitial "noise"). That's where the $10,000 (okay, so prices have started to nudge a little lower, but $7-8,000 isn't exactly "pro-sumer") camera bodys come in. Low noise ISO 3200 isn't cheap, especially when you want to be able to save several fps in long bursts at high resolution RAW or TIFF format to the multi-GB micro hard drive inside your camera. If you'll notice, most of those guys also have the entire camera/lens system mounted on either a monopod or even a tripod, or securely clamped to something solid like the stadium itself. They are using IS lenses that sometimes cost more than the camera body (see above) they're hung on to get that fast moving athlete frozen in time (instead of slightly blurred) at just the critical point in the key play of the game. Assuming they guessed correctly and pre-focused and metered on the correct spot of the field before the play actually got there. If they do, it might be worth whatever the cover of next weeks SI or Sporting News is paying these days. But they don't buy IS lenses so they can leave the monopod at home. They buy IS lenses so they can get more out of the entire combination.
Ultimately, like most questions along the lines of: "Is ____ worth the extra cost?" when talking photo equipment, the answer depends on what type of photography you want to do and are capable of doing.