I don't think you can be that general about any one brand across their entire range. Several lenses I'd come across in the shop may even be badged as different makes but be the same lens!
One or two Sigma's were awful, Canon, Pentax, Nikon and KonicaMinolta/Sony have also had their fair share of duds, as well as crackers.

Must say that Tamron as a manufacturer isn't as prominent in the UK as they once were. Some Tamron lenses are far more common in the states I believe.

Going for any lens which isn't from your camera manufacturer is always perceived as a 'risk'......
To be really honest though unless you go for a real cheapie (cosina for example) most lenses made today are made to a very good standard, even compared to one's from yesteryear.

Got mine for £126 brand new, they have to reduce prices a lot more for Pentax fit as it's harder to shift. ;)

Agree that it is a stunningly good quality optic.

The same happened with the KonicaMinolta fit option too. Not sure if it's still the same now the mount's been badged Sony.

Look for a used Canon 17-40 f/4. I find this lens very sharp and works well in low light conditions dispite it's relatively slow f/4. You may be interested in this as well

-ex-dg-macro-af.htmlhttp://www.popphoto.com/cameralenses/3407/test-sigma-70mm-f28