I used the Sigma for my first few years as a photographer, suitable for entry level work. Absolutely no comparison to the Canon glass, much less consistent auto-focus and a lot softer images. But it works, and most can't afford the Canon glass.
If you are going Canon 70-200, spring for the image stabilization. It is more than worth the money. You will come across times where the lighting requires a very slow shutter (< 250th of a second), and hand-holding below that results in soft/blurry images. But with the image stabilization, you can hand-hold as low as 1/80th of a second. Today inside a poorly lit, yellow-fluorescent barn, I had to hand-hold at 1/100th of a second to get useable headshots, but they turned out quite fine.
One big tip for anyone looking to shoot indoor sports, get a Canon 85mm 1.8 lens for about $300. Most indoor arenas are not well lit enough to use even 2.8 lenses without ISO 3200 (which usually only comes on the pro-level cameras). It's not the fastest lens but enables you to get shots you could not otherwise. Also, it's an excellent portrait (headshot) lens. Don't get the 2nd version of it which is over $1000.
:2004::moo:
