I've been using a Sigma 150-500 with my D7000 for over a year now and think it's a great lens for wildlife. I do a ton of backyard birding and it's been perfect. My
Flickr Photostream is chock full of that combination if you want to take a look. Most of these were taken handheld as well, though 1/2 the owl shots were on a monopod.
That said, my brother (who shoots Canon) just picked up the Sigma 120-300 f2.8 and I'm starting to wonder if that'll be my next lens. He's a newspaper photographer and has used it for sports, with and without a 2X converter, and he loves it. Rugged lens too - in the middle of shooting US Open Tennis it fell to the ground face first when his monopod lost its balance against a rail. While I wouldn't recommend this test, the lens hood absorbed enough of the shock that while it needed repair the lens didn't (and the crash was loud enough to get a rather ugly glance from Andy Murray).
The one thing I'd say about the 120-300 is that it's significantly heavier. Would be a tough haul out in the woods and you're going to need to go into weight training to handhold it. But the idea of that and a 2x next time I'm out somewhere shoot in the wild is very tempting.
And for what it's worth, Sigma service is excellent. The AF motor on the 150-500 stopped working after a year and they repaired it with a 10 day turn around. Can't beat the warranty.