I've been using this lens since January. If you really never go below 1/250th and nearly always use a monopod then I think you don't necessarily need IS. But at 1/100th I would say the IS is still useful. For slower shutter speeds its outrageous what you can sometimes get away with handholding!
Of course, the old rule of thumb is you can reliably handhold a lens and still get a sharp picture at a shutter speed equivalent to 1/focal length. eg. if you have a 100mm lens then you could get away with 1/100th sec but its touch and go with slower speeds. I've always found I could handhold slightly slower speeds than this rule but it all depends how steady your hands are. Nowadays it also depends how closely the pixels are packed into your sensor. The 20d is more prone to shake than the 1d mkII for example because its pixels are more densely packed onto the sensor (same number of pixels on a smaller chip). So to some extent it depends on the camera too.
All the 70-200L lenses in Canon's range have a great reputation. I can definitely vouch for the sharpness of the 2.8 IS.
While I haven't done a proper test, I did have some results from a 300mm f4LIS that weren't as sharp as I thought they should have been. These were taken at about 1/160th resting on a beanbag. I suspect the softness in the images came from the IS operating on a picture which was essentially already steady. The 70-200 has a later version of IS which I think is supposed to cope better when tripod mounted but I turn it off on a tripod. Both these lenses have 2 IS modes: one for handheld where shake can be in any direction and one for panning where shake is only expected in one plane. Or of course you can turn the IS off. I do a fair bit of photography on boats. The IS has definitely proved its worth there.
So to sum up. On a monopod at 1/250th I don't think you will see much benefit from IS and I do think that it may actually degrade the image. If you handhold at 1/100th sec with the lens set at 200mm the IS will probably improve sharpness. If you handhold with the lens set at the wider end (70mm) shake might have been fairly negligible anyway so you might not see much benefit, If you use a tripod, then unless your lens is 500mm or more when shake can be a problem even on camera supports, then do not use IS.
