For me, this is the ultimate lens. I came from a Canon 70-200mm F2.8 which I found too heavy to carry around. This lens is much smaller, lighter, cheaper, and it's coloured black. Its performance is also slightly better, resulting in sharper images wide open. However, don't buy this lens over the 70-200 just for the image quality, because both are superb and there's not much to tell between them.
If you're looking to pick up this lens second-hand, see if you can find a good copy of the discontinued mk1 version. This one's optically similar to the mk2 but has a built-in lens hood. For me this is ideal, because the lens hood is stronger and smaller than the bulky hood that comes with the mk2. Also, you can't lose it. What surprised me is that I could attach 77mm filters to the hood (though rendering the hood useless) but also my 77mm lens caps - handy if you like. I have a tripod ring for the 70-200mm F4 which also fits this lens like a glove.
I have also used the 70-200mm F4 and the F4 IS as well as the Sigma 70-200mm F2.8 and ultimately found that I only use the long end of the zoom. So for me it only made sense that I'd go for a prime. And I got it CHEAP.