I concur 110 percent. In fact, I am just about to give up my 24-70 f2.8L for a 24-105 f4.0L. I am not happy about loosing the f-stops, but the IS really is that big of a deal. And of course, this is yet another reason why I (along with the rest of the planet) want to see either a 24-70 f2.8L IS or make the 24-105 an f2.8.
Truth be told, I really didn't think the IS was that big of a deal, until one day I was messing around with my 28-135 that came with my 40D kit a couple of years ago. That lens has IS and boy could I see the difference...and I wasn't even looking for it. So that sold me right then and there. And yea I know (before someone brings it up) there is an IS f2.8 lens in the EF-S line. But I refuse (and its just a personal decision for me) to buy EF-S lenses. Its a future upgrade thing for me.
Please please please Canon make it happen!!!!!! Maybe next month eh? I hear rumblings about a couple (maybe three) new lens announcements next month. Who knows though...sure would be nice.