My brother, a Sigma head, pinged me with this yesterday. At first I was rather dismissive, but the more I read the more I understand it. The weight is a concern until you consider this - this lens is targeted at pros, and pros don't shoot sports handheld. Period. Really - how often do you see guys lugging 300mm f2.8's around on their shoulders on the sidelines of games? These things are anchored to a monopod, and you know what, they should be. 600mm on a 24MP DX sensor is going to require you to shoot at about 1/1250s to get a really sharp photo, and faster if you're doing it handheld. How often is that going to happen?
Here's what I hate about this lens.
1. The cost.
2. The speed.
3. The weight.
Here's what I love about this lens
1. The focal length lock. Having used the 150-500 for years, I can't tell you how often minor focal length creep has softened a photo. This is truly a pro feature. Lock the length, concentrate on shooting.
2. The USB dock. Nikon lets you fine tune a one point per zoom lens, Canon lets you do two. Sigma overrides that and will tune to 4, albeit for only one camera at a time. Tune this sucker to your D7100 and you're set.
3. The construction. It's big, it's heavy, and it's meant to be used outdoors, so it needs to be weather sealed the way it is.
4. TC compatibility. Granted, you need to buy Sigma's, and it's going to AF at f8, so it may be troublesome in the dark. But a 210-840mm (effective 315-1260mm on the DX) is significant for wildlife.
I'm going to be waiting and watching on this one. It's a hefty price, but if the IQ allows me to shoot at one or two clicks closed rather than at f9 it's going to be well worth it in the long run.