Although some compare the 70-200 to a Sigma, there're differences to be noted. As many say, it's an L and resale is much better than any 3rd parties. That's just the start...
The f/4 is a sharp lens, optically similar to its 2.8 brother. IMO, yah, I wish this lens had an IS. But then, it depends on your shooting style. Some amateurs go straight for the 2.8 version. Well, if you really need it, fine. But most of the time, I'm shooting at f/4 in good light and handholding is no problem. I wouldn't want to carry an f/2.8 "rock" around when I travel. It's buky too, attracts far too much attention.
The Sigma loses out in terms of focusing speed and distance too. You want L, you pay L price. But I can tell you you won't regret it.