It is a very good lens
I purchased the 120-300 f2.8 OS APO EX DG HSM (alpha soup) lens in early February/March of 2011 before a trip to Yellowstone. Prior to receiving this lens, a Canon 70-200 F2.8 IS was the longest lens in the bag. Based on my experiece in Yellow Stone, Hawaii and numberous polo matches and other equestrian events, I've come to appreciate the lens but also better understand its limitations. Weight is not a factor for me. In fact, when shooting hand held I do better with a heavier lense, so it's not a problem. However, most of the time I shoot from a tripod using a Manfrotto 393 head and often times us a cable release to minimize vibration. The ability to quickly go from 300mm down to 120 mmm and then back up to 300 is a real benefit when shooting polo from behind the end zone and the action is around the goal. Polo is fast and one must be able to adjust quickly to the fluid nature of the action. 300mm is still to short for many wild life encounters, but I am able to use the Canon 1.4x multiplier to increase the effective focal length, which came in very handy when capturing images of a male Osprey returning to his nest with a fish "in hand" for his mate and chicks. The only error I made on the trip was on day 2, when I mounted an inexpensive polarizer filter (105mm) and all of of the resulting were all out of focus. All of the grizzly pics I took (using a tripod and the cheap polarizer) were included in the carnage. After one day's use the filter ended up in the trash bin. I learned a very hard lesson that day and have since purchased a much better polarizer, but purchasing a decent 105mm polarizer is costly. As a side note... I've noticed that when ever I show up at an equestrian event with the Sigma 120 to 300mm lens, folks assume I'm a pro and treat me accordingly, but then I do take photography serioulsy and enjoy the respect these people show me. I have a weakenss as well. I really love it when people comment on the size of the Signa 120-300mm lens I'm using because I always get the last word in when I tell them "size does matter". ...and it does. Now I'm yearning for a 500mm lens. Will I ever be satisfied...
This afternoon I sent the 120-300 off for repair. Several times over the last year the lens has malfunctioned and each time it quickly self corrected. Last Sunday when attending one of the last polo matches of the fall season it happened again, but this time the problem did not correct itself. I contacted Camera Repair Japan in Duluth, GA, an authorized Sigma repair center and described the problem. The individual that answsered the phone was knowledgable enough to explain there are a couple of things that could cause the problem I experenced, but until they actually look at the lens they won't know for sure. $68 to ship it UPS ground to the repair center and then an estimated $150 to $250 to repair the lens isn't the issue. The issue I have is this lens has been babied. There is no way I would expect a problem with this lens after owning it for a year and a half. Early next week when the service center contacts me I'll find out what the problem actually is... as well as what it is going to cost me. Timing is not good. In three weeks my family and I are leaving for Costa Rica. I am hoping to have the lens back and in working order by then. My fingers are crossed