Thoughts on SIGMA 17-50mm LENS F2.8 EX DC OS HSM?

Thinking of buying theses lens for NIKON D3100(CROP SENSOR)
What do you think about it
Is it mostly suitable for landscape photography(but my dslr is crop factor so i'm guessing is not that great for landscape)?
I want lens for street, people photography?
will SIGMA 17-50mm LENS F2.8 EX DC OS HSM do?

Good lens for just about everything you could do with your kit lens. With your DX camera, the wide angle is like a 25.5mm, which is still pretty wide without going to a specialized wide angle like the 12-24.It is brighter than your kit, and with a constant aperture is gives you 2 more stops of light at the long end.

Unless you want to shell out the cash for the Nikon 17-55 f/2.8, this is about the best kit replacement you will find.

You may also wish to look at the Tamron 17-50 for comparison purposes.