You don't need Nikkor or "L" glass to have good results.
If you can't afford it, and don't need it, don't get it. You'll do perfectly fine with the Sigma. If you read reviews it comes out being a pretty good deal for the money.
There is a difference in sharpness and build quality, but it all depends on what you need. I have two "L" lenses, the 24-70 2.8, and the 70-200 2.8. I use them because I take them where I want them to hold up. They are always in a backpack sledding, dirt biking, and on my motorcycles and hiking. They have to survive lots of vibration and me falling on them when I fall of.
I also have a 10-22 EF-S lens which is not the L build construction, but it's one of my favorite lenses and the quality and IQ is still quite impressive.
With anything in life, you have to consider your needs and wants. If you need it, save up, if you want it, and don't need it, then consider the alternatives.
I use the Sigma 70-200 for Nikon and love it. Yeah I would prefer to have the Nikon 70-200 but its about double the price. The sigma is very sharp and I would reccomend it to anyone.
I was gonna go with the sigma but went with the Nikon at the last second for fear of making a mistake and not getting the very best. If I had more time and made a effort I would see about renting both and test them out. But that may not be possible. Oh yeah I love the Nikon but I never shot with the sigma so I will never know and I won't want to know now! lol.
I've got a D200 with the Sigma 70-200 and 2X teleconverter.. just picked it up This weekend.. I haven't had much chance to test it out but I'll see if I can give oyu some examples of sports shots