If you can afford it, IS is absolutely beneficial. Maybe not exactly for sports shooting, as you need fast shutter speeds anyway, but for anything moving somewhat slowly, you'll notice an improvement by leaps and bounds. If you're into trying some panned shots, IS will help you out even in the sports shooting department.
I can hardly get a sharp image at 1/125th at 200mm without IS, but I've had good keepers at 1/25th at 200mm before, hand held with IS! At 200mm, I need 1/200th second to be safe without IS, but really I only need about 1/50th to get my shot with IS. In low light this will save your life.
For walk-around hand-held shooting, it's halfway to having a tripod. It's really that good.
While IS may be good for the longer focal lengths when you cannot shoot at higher shutter speeds (which is rare with DSLR as you can always up the ISO) it most certainly is not needed for shorter focal lengths like the 24 or 28 mm lenses that just came out with IS.....CANON.
Nothing like adding another $300 to the price of a 24mm or 28mm lens by adding an unnecessary feature that can fail (IS) to a short focal length length that would not benefit from the additional feature.
Just because you CAN build IS into a lens does not mean you SHOULD, Canon.
Have you ever compared an IS to non-IS lens when shooting video? The difference is ridiculous. Any sort of shake makes video very hard to watch. Canon is clearly not all about stills... video and cine is a crucial part of their business plan.