A lens like this will be well-used for outdoor sports, wildlife/ birds (together with a 1.4X TC), events, and maybe also portraits, but I reckon the working distance might be a little prohibitive for the final application.
It's definitely a good lens, rated very highly at
www.fredmiranda.com and got a stunning review at
www.photozone.de
I think it's good value for the money considering the focal length and the moderate maximum aperture, plus the IS, which is undoubtedly handy for handholding. Ask yourself if you want a prime or a zoom.
Also, for sports (including indoors), some might recommend the 70-200mm f/2.8L (IS or non-IS). For portraits, you might be better off using an 85mm f/1.8 which costs just around $600. If you're using a camera such as the 300D, 350D or 20D, or even the older D30 and D60, the 300mm lens becomes the equivalent of a 480mm lens, and you'll be hard-pressed using this indoors for portraits. Not to mention that the aperture of f/4 is a little tough to use for indoor sports as well, even if the interior is large enough to warrant the use of such a long lens.