Tamron vs Nikon - image quality?

If anyone has any experience with various lenses of Nikon and Tamron, is Tamron even in the same level in regards to picture quality and sharpness? I have my eye on the Tamron AF 18-270mm f/3.5-6.3 Di II VC PZD LD. Does anyone have this and have any opinions?
I have a Nikon D3100 and I'm ready to upgrade from the Nikkor 18-55mm.
I want really versitle lens, a good walkaround, but takes great quality photos. Better quality than a normal point and shoot.

I've used the 18-270 and was surprised at how decent it was - I was expecting it to be terrible. As long as you're not shooting sports or in low-light situations, its a decent walk-around lens. Build-quality was OK, auto-focus was faster than I thought it would be (even though it was still slow) The Nikon 18-200 is better, overall.No, Tamron lenses are usually not as sharp as genuine Nikon lenses.http://www.dpreview.com/lensreviews/tamron_18-270_3p5-6p3_vc_n15/

Tamron exists because Canon, Nikon, Sony and Pentax are expensive. Now in photography, you get exactly what you paid for.

Another fact of life is this. The longer the zoom range of the lens, the poorer is its optical quality. Blame science on that. You can have versatility or you can have optical quality but you can't have both.

If you know exactly what you are doing, the 18-55mm lens can be the only lens you use for the rest of your life. That is if you know exactly what you are doing. That's because picture quality depends on the photographer's ability. The camera and lens are only tools.