Although I’m a big fan of Sigma lenses, the Sigma 18-50 3.5-5.6 is not one of their best. It’s OK, but although it’s better made than the Canon 18-55 EFS, if I had to choose, I’d rather put up with the plastic Canon product.
The Canon 18-55 EF-S has very high central sharpness and is perfectly useable overall at f8 on, whereas the Sigma 18-50 DC has better edge performance, but is nowhere near as sharp in the center. The newer Canon 18-55 IS is slightly better optically so is even further ahead. So why buy the Sigma?
Performance is quite similar to the Sony / Minolta 18-70, as in not that good, so that’s not really a compliment, and for Nikon buyers It’s not even close to either the build or optical quality of the Nikon 18-70mm, or 18-105 DX.
To be fair to Sigma this isn’t an expensive lens, and for snapshots on the beach and small prints, it would be fine, but it’s not better, or cheaper than most kit lenses. My advice would be to stick with the manufacturer’s kit lenses - especially if you’re buying Nikon where their 18-70 DX is where your cash should go.