I have a Sigma 50mm f1.4 which I ( and the reviews) reckon is better than the Canon equivalent, & a Sigma 85mm f1.4 which also delivers great results. Interestingly the Tamron 24-70 VC gets better reviews that the Sigma & appears to be almost on a par with the Canon which doesn't offer IS/VR
So as Gordon says - dont knock 3rd party lenses - read the reviews carefully - so of them are relative bargains