Is 17-50 2.8 from sigma lens worth buying?

I have nikon d3100 (crop sensor camera) looking for wide angle lens but already have kit lens which i'm not sattisfied to use them as wide angle. I want something wider so what about 17-50 2.8 from sigma. I think is not much difference from my kit lens but want to read your thoughts

The benefit would be that you could stop it down further to keep even better depth of field or use it in dimmer lighting conditions. But it isn't what I would call wide. For me that starts at around 14mm and there's a Samyang designed for just that… F2.8 as well. But It's manual focus and so won't be happy on your Nikon. I'd get a 10-20 Tamron or Sigma.
See the Dyxum review listings in the link. Don't worry that it's Sony based… You'd just get that lens in Nikon fiting if available.
http://www.dyxum.com/lenses/resultsAdv.asp
Click the lenses tab at the top and choose zoom lenses. Wide angles are in the first pages.

There are some very good reasons to buy the Sigma 17-50mm, but unfortunately, you will not likely see any difference in the wide angle end. One mm is not even going to be noticeable.

If you want a super wide angle lens, consider the $600 Tokina 11-16mm f/2.8 DX2, which is a highly regarded lens, and probably the best lens super-wide angle lens for Dx cameras. But be sure you buy the more expensive DX2, as the original DX does not have a focus motor, which means it will not autofocus with your camera. The difference between 11mm vs 18mm is huge.

Regarding the Sigma 17-50mm. This lens is another one of the most highly regarded DX lenses you can buy. It beats Nikon's 17-55mm f/2.8, and the Canon version of the Sigma 17-50mm f/2.8 also beats Canon's 17-55mm f/2.8; so says DxOMark.

The fast f/2.8 of the Sigma (as well as the Nikon 17-55mm f/2.8) is vastly superior to your kit lens. And since the Sigma ($600) is half the cost of the Nikon ($1, 200), it is one of the few Sigmas worthy of owning, and I highly recommend it.

However, Sigma's biggest problem is quality control. And for this reason, I recommend buying the lens from a camera shop rather than on-line so you are assured of a good copy.

Why are these lenses an upgrade over your 18-55mm kit lens? They are pro-caliber optics, with a fast constant f/2.8.

And here is why that is important.

At 55mm, your kit lens has a maximum aperture of f/5.6. That pretty much makes it a daylight-only lens. But more importantly, the Sigma's (or Nikon's 17-55mm f/2.8) f/2.8 aperture at 50mm is 2 full stops better than your kit lens.

Two stops are huge, and you just can't replicate that performance advantage in a better camera. Even the $6, 000 Nikon D3s, which is still the best low-light DSLR is only 1 1/2 stops better than the D3100.So by spending $600 on a lens, you get more low-light performance than spending $6000 on an new camera.

That is why lenses are so important.

A 2 stop improvement means you will be able to use lower ISOs. Say you have to use ISO1600 in a low light photo with your lens at f/5.6, By using a f/2.8 lens, you can lower the ISO to ISO400, which will vastly improve noise.

And with fast action and sports shots, it can make the difference between sharp and blurry photos. If your maximum shutter speed is 1/60th with a f/5.6 lens, you can use 1/250th with a f/2.8 lens.

And finally, no lens is perfect - even those costing $20, 000. All lenses become sharper when stopped down a bit from their maximum aperture. So a f/2.8 lens stopped down to f/5.6 will provide sharper photos than your kit lens at f/5.6.

Now, there's nothing wrong with your kit lens. And in fact, some people use it without ever having to replace it. However, it does have a few limitations, and if you want to take your photography to the next level, consider buying higher performance lenses.

An alternative is to buy a prime (non-zoom) lens; either a 50mm f/1.8 or a 35mm f/1.8. While they will not have the convenience of a zoom, they are even faster than the f/2.8 (1 1/3 stops faster than a f/2.8, and 3 1/3 stops faster than a f/5.6). And they are less expensive, around $200 for each.

Now, with a prime, you will have to learn to "zoom-with-your-feet" to get the zoom range of your kit lens, but this is easy enough to do with a mid-range focal length as most of your photography will be close up (as opposed to super wide angle or telephoto lenses).

If I were to outfit a new Dx camera, this would be my "holy trinity" of the three lenses I would get for either Nikon or Canon:

Tokina 11-16mm f/2.8 Dx2
Sigma 17-50mm f/2.8 (or Nikon 17-55mm f/2.8 or Canon 17-55mm f/2.8).
Sigma 70-200mm f/2.8 (or Nikon 70-20mm f/2.8 or Canon 70-200mm f/2.8).

Notice a commonality? All of these lenses are a constant f/2.8, which is considered pro-level.

And don't worry about the gap between the 50mm mid-zoom and the 70mm telephoto. You don't have to cover every millimeter, will not notice that gap at all.