Like the post says i have a D300 and i just purchased a sigma 120-300 2.8 OS few days ago. So far i'm happy with the lens. It focuses accurately and fairly quickly, but not blazing fast. It does seem to have a little bit of trouble in AF-C mode when moving from infinity to close focus, however, so does my old 70-300 VR, so it's possible it's a body problem rather than lens problem. I actually have no trouble with the weight at all as far as shooting. I shot it handheld for about 2 hours the other morning and had no problems. The funny thing is I find the 120-300 easier to handhold and keep steady than my 70-300. The heavier weight keeps the body and lens from jiggling as much i guess. It is strange having to always keep one hand carrying the lens. It makes it difficult to press the ISO and WB buttons and roll the dials. I had to setup custom menus to do this more easily which kinda sucks, but this is no fault of the lens itself. all praise being said, 3200 is a lot of money to put into one lens, especially one with only above average AF speed and that I can't find a shoulder bag to fit it into.
My question for you guys is more of an opinion based question, but in my situation, would you keep the 120-300 os with the d300 (since now i can't afford another body for a long time) or upgrade to say a d800 which allows me to crop to higher Mpx than the D300 at same FOV, better ISO, better AF and such, and get something like a Nikon 300mm F4. I realize i lose a stop with f4, but really gain two stops of clean ISO back with the d800, and I'll still be able to autofocus with the 300 f4 with all TCs should i use ever decide to use use them. Plus i can more easily carry my gear. It's a dilemma because i do really like the 120-300 OS. Thoughts?