The Sigma APO 120-300mm F2.8 EX DG OS HSM is a unique lens. There are no lenses of similar range and speed offered by any other lens maker. The nearest thing offered by most is probably a 70-200/2.8, but if you need more telephoto reach than those lenses offer, your choices are limited and pretty much restricted to 300mm prime lenses.
There is of course the Sigma 200-500mm F2.8 APO EX DG but at $26,000, for most of us at that price it might as well not exist. The 35lb weight makes it a tad impractical too.
Sigma themselves have an APO 300mm F2.8 EX DG which about 3" shorter and over 1lb lighter , but it doesn't have optical stabilization. With a street price at $3400, itís $200 more expensive then the 120-300/2.8 zoom ($3200).
Canon have their new EF 300mm f2.8L IS II USM which is also shorter and lighter (5.0 ◊ 9.8 in, 82.9 oz. / 128 ◊ 248mm, 2350g), but a lot more expensive at around $7300 - almost $4000 more than the Sigma APO 120-300mm F2.8 EX DG OS HSM.
Nikon also have a 300mm f2.8G ED-IF AF-S VR II prime which is again a little shorter and lighter than the Sigma, but at around $5900 it's over $2500 more expensive.
Are the primes better? I don't know, but given the additional cost of the Nikon and Canon lenses and the lack of OS on the Sigma lens, the Sigma APO 120-300mm F2.8 EX DG OS HSM starts to look like quite a bargain!
I was pleasantly surprised with how good the Sigma APO 120-300mm F2.8 EX DG OS HSM turned out to be. Given that it's (a) a zoom, (b) a 3rd party lens and (c) less expensive than any other 300/2.8 on the market my expectations were modest, but in fact it turned out to be a very good lens indeed.
Used on it's own it's very sharp at all focal lengths and apertures in the center of the frame and still pretty decent all the way out to the corners of a full frame image. Distortion, vignetting and chromatic aberration are all very well controlled and AF was quiet and accurate.
Performance dropped a little with the Canon 1.4x and 2x TCs, as you would expect. The images were still very usable (especially with the 1.4x), though not up to the quality of an equivalent prime lens.
For applications such as indoor sports photography the fast aperture and ability to zoom would make the Sigma APO 120-300mm F2.8 EX DG OS HSM an ideal lens. It's also very usable (in fact very nice) for outdoor portraits (or indoor portraits in a very big studio!) and outdoor sports where you are fairly near the action. It can be handheld if necessary, though at 6.5 lbs it's no lightweight and a monopod makes life much easier. 300mm is generally a little short for wildlife, especially small wildlife like birds unless you are at zoo, but it's certainly usable, especially with the lens on a crop sensor camera which gives you the effective 1.5x (Nikon) or 1.6x (Canon) digital multiplier.
Someone is bound to ask if it handles high speed AF tracking as well as a lens like the Canon EF 300/2.8L IS II USM does and I'm afraid the answer is that I don't know since I didn't have the Canon lens on hand to compare it with. AF tracking of moving cars with the lens used on an EOS 7D was good, but not perfect, but then again AF tracking is a function of the camera as much as the lens.
My conclusion is that if you want a 300mm f2.8 lens, then the Sigma APO 120-300mm F2.8 EX DG OS HSM is a relative bargain at "only" $3200. That fact that it's a 120-300/2.8 zoom just sweetens the deal. However if what you really want is a 600mm lens but you are considering using a 300mm lens + 2x TC for most of your shooting, I'd really recommend you look at a longer prime.
If my budget was limited (as in limited to under $4000!) and I wanted a 300/2.8 lens, the Sigma APO 120-300mm F2.8 EX DG OS HSM would be at the top of my list. Even if my budget wasn't that limited, it would still be on my list because of the added utility of the zoom function.