As a 50-150 non-OS user, here are my thoughts:
As Jim commented, the choice should not be based on the release date but on how you use the lens. Yeah, it's annoying Sigma keeps pushing back the release date for this lens. On the other hand, if the OS lens is what you really need, and you get the non-OS version now, then once the OS version is released, you may feel pangs of regret for the rest of the life of the lens. Besides, you'll be using the lens for many years, so a few months' delay is not significant compared to the utility of the lens over its life.
Based on my usage of the 50-150 non-OS, I do think having OS would be helpful and would make the 50-150 lens much more useful. However, sometimes I don't need OS:
- One example that Jim has pointed out is if your subject is fast-moving, and you want the subject to stay sharp, you'll need a fast shutter speed anyway and despite having OS, you won't be able to shoot at a slower shutter speed.
- In moderately bright ambient light, OS isn't a necessity. In the morning or afternoon, with an ISO of 400 or 800, I can usually get shutter speeds of 1/500 or 1/1000 or faster. At those speeds, my images are as sharp as my lens will allow and camera shake isn't a significant factor for me. See: Why I love my Sigma 50-150.
- In dim ambient light when I can use flash, OS isn't critical either. The flash duration (which is 1/1000 or faster) becomes my effective shutter speed.
- At high ISOs, you may be able to get high enough shutter speeds in somewhat dim light. Please note: I'm not afraid to use high ISOs. I often use ISO 1600 with my D70 and use ISO 3200 with my D300 when I have to. It helps that I use Lightroom 3 which has pretty good noise reduction capabilities. In the end though, I'm just not a pixel peeper. I don't mind luminance noise, and LR3 does a pretty good job of getting rid of unsightly chroma noise. Yes, some details do get lost with high ISO and noise reduction but my audience doesn't make large prints. They look at images on a computer monitor at up to 1600 pixels on the long edge -- that's like a 2-megapixel camera resolution. So I personally don't have a strong need for very crisp and detailed images.
Notwithstanding the counter-arguments I discussed above, I am still tempted by the 50-150 OS:
- Dim ambient light. Sometimes, raising my ISO isn't good enough and flash is not an option.
- Dim ambient light even with flash. Even when I can use flash to freeze the subject, I still need a slow shutter speed to allow the background to "burn in". If you don't have OS, then the background will be somewhat blurry due to camera shake. If you prefer a sharp background, then OS would be needed even with flash.
- The new lens is a total overhaul of the 50-150 non-OS. The new lens has 21 elements in 15 groups, compared to the non-OS lens which has 18 elements in 14 groups. The new lens also has a huge filter size of 77mm compared to the 67mm of the old lens. I'm guessing that Sigma didn't just simply add OS to its old lens and instead tried to improve on image quality. I would not at all be surprised if the new lens turns out to be significantly sharper than its predecessor.
One comment that I sometimes hear from prospective buyers of the 50-150 is that you may as well buy a 70-200, particularly if you're going full frame. I'm one of those folks who are thinking of possibly getting a full frame camera in the future. However, if I had the choice of a crop-sensor body with the 50-150 or a full-frame body with a 70-200, with all factors being equal (e.g. image quality, sensor noise, etc.) I think I would go with the 50-150 because of the depth of field. With my 50-150 on my crop-sensor body, the depth of field is just enough to render the subject sharp from nose to ear while still having a blurred background. See: Why I love my Sigma 50-150. I don't have personal experience with a 70-200 on a full-frame camera but I have seen some professional photographers whose 70-200 images (on full frame cameras) had insufficient depth of field. It's just something to think about...
If you ask me if I plan to upgrade to the OS version, the answer is probably not. The reasons?
First is the size of the new lens. It's about the same size as the 70-200 OS (7.8 inches long) compared to the current 50-150 (5.5 inches long). It would be inconvenient to bring it around, and if it's inconvenient, I probably won't use it as much my current lens.
Second is the cost. To get the new lens, I would need to sell my 50-150 (probably take a $50-100 hit), then put in additional funds to buy the OS lens (probably another $500 hit).
In my opinion, the new lens would have to be really amazing for me to upgrade. YMMV.
If the most important factor for you is image quality and you want the best image quality possible from your crop-sensor camera at this focal length range, then you probably should wait for the 50-150 OS.
If you are an aspiring pro or may be getting a full frame camera in the future, you *may* want to consider getting a 70-200 OS instead. Meanwhile, to cover the 50-70 gap, you may want to use a 24-70, 16-85, or the Sigma 17-70 for your standard zoom. But note that the depth of field may be shallower.
If you are care about things like convenience and portability along with image quality, then you might not be a pixel peeper, in which case the compact 50-150 non-OS may meet your needs better than the 50-150 OS.