That is a sweet setup! And it's doing a fine job from what I see. Antelope are always a tough big game animal to get great photos of because in a lot of areas they just don't let you get very close.
My setup right now is a Canon XTi with a Sigma 50-500mm. So far, so good! The only problem I've had so far is when I saw a bear standing 30 yards off the highway just north of Morenci about 10 miles. I pulled out the camera and tried to snap some shots, but because of the brush around and the limited light (early morning) the camera wouldn't pick a spot to focus and never focused on anything. I'm not sure if that's a limitation of the Sigma or just of the available light with a 500mm lens. Either way, for everything else I've photographed it's worked great (SO much better than a fixed lens camera!)
Good luck this year getting close to these animals and taking pictures!