Sigma lens vs. Nikon lens?

[justlearning]

I am not a professional by any means but I would love to be someday. I have a Nikon D90 and just bought a Nikon D300s (I was having problems with the D90 and thought I would try something else). Anyway, I have a couple of basic lens but the one I use the most is my Nikon 18-200 (the newest one) and like the lens but I really wanted to get a Nikon 70-200 2.8 VRII but being that it is over 2K I don't think I can afford it anytime soon.

Sorry but my question is, what do we think of the Sigma 70-200 2.8? It is about a third of the price but I wasn't sure if it was even worth me going for it. If it isn't a good enough lens than I should probably just stick with the 18-200 that I already have.

Does anyone use one that might have used the Nikon one that they could tell me what the differences are? Is it sharp? Should I just wait?

Any feedback would be appreciated, thanks