also, for those on more of a budget, but wanting 2.8, the 80-200 2.8 D lens is still sold brand new, for just over a 1000 dollars versus 2300 for the 70-200 G lens. Build quality of the 80-200 2.8 is great, and what you give up is the VR, but still, a Pro quality lens for 1000 dollars!
Used, I have seen these around 800 dollars. but at a 1000 dollars I would seriously consider buying brand new.
Both of these lens are tanks, so,, expect to develop muscles carrying around. I tend to use this lens when I am at an event where less mobile, and mounted on a tripod (which negates the VR anyway).
Update -> turns out that the 5100 will not focus this camera, so you have to have a 7000 300, D3, D700, or D800 to autofocus. I thought the D5100 had a lens af drive? but talked to someone that just bought the lens, and she had to manual focus it on the D5100. Can still use the metering, etc, just not autofocus.