How good is Sigma's 70-200mm F2.8 EX DG APOMacro HSM IIlens?

I'm wondering why is it half the price of nikkor's lense! Is it because Sigma hides a very big BAD secret?

It's half the cost because it lacks the VR (vibration reduction) that the nikkor has. Not too long ago, Sigma announced the 70-200mm f/2.8 EX OS (stabilized) lens http://www.dpreview.com/...p8_os_c16/, which is about the same price as the nikkor. Technology costs, and most people overlook that fact. No secret here, bro. Aside from that, the unstabilized sigma gets pretty good reviews, and is optically similar in performance. But understand that sigma has to reverse engineer the autofocus algorithm used by nikon, which is proprietary. This means that it may not focus as accurately under similar conditions as the nikkor, which is usually the biggest complaint of third party manufactured lenses. So you take a risk, but for many, the risk pays off. But really, only with the non pro glass. You'll find that faster, pro "EX" model sigma lenses are about the same price as nikon glass, if not more in some cases (example being the sigma 50mm f/1.4, which is about $90 more expensive than the nikon 1.4 AFS version). Keep that in mind.

I've never been a big fan of Sigma lenses. They are OK, but the Tokina lenses are better built.
I've owned a few Sigma lenses, they just don't have as solid of a feel to them as a Nikon or Tokina lens.

The Sigma 70-200 is about the same price as a used Nikon 80-200 2.8D 2 ring, 725.00.

The Tokina 80-200 2.8 goes for around 350.00 on Ebay.

With Tokina, Sigma or Tamron, they're rarely going to be as good as Nikon or Canon lenses. For 1/2 the price though, they are only a few % points down on optical quality, so it's a good deal overall.