Okay, I haven't read the whole thread because it's very long. I made it to about page 6 I think before I gave up and jumped to the last couple, so pardon me if this point has already been made.
One of the recurring themes in the thread has been, my Sigma lenses are very very sharp/sharper than my L lens/as sharp as my L lens, etc. The problem is, barring potentially the new Nikon D2x, all other DSLRs should be the resolution (sharpness) limiting factor, not the lens, unless the lens is *really* really awful. So unless you shoot with something other than a DSLR, and then view your images at a respectable magnification (8x12 is not exactly large), you're not going to notice the resolution difference in *most* lenses. You will get the odd few, really poorly made, or major budget (unknown) lenses, or lenses with problems like coating loss or fogging, but anything from Sigma, Tamron, Canon etc will do the job for the vast majority of purposes.
Secondly, this is a link I originally posted in late 2003 that I dug up because someone asked me about macro lenses recently. In no way do I have any vested interest in that article, nor can I vouch for its accuracy, but I don't find any problem with the guy's methodology nor can I find any reason why he might want to doctor the results (aside from a smear campaign). Those of you who think Sigma are necessarily all the way up to scratch, have a look at the link
here.
I'm not saying the Sigma 70-200 isn't any better than the Canon or Nikon equivalent, it might well be. Even if the linked results are true, just because Sigma makes a couple of duff lenses doesn't mean they are not capable of producing other good lenses.
But think carefully because you say your 70-200 Sigma is good.
Have you really pushed it enough to be able to say that yourself
(see above)?
Have you a decent means of comparison to be able to say it is good
(ie something else that is worse)?
Or have you heard someone else, whether online or in person, tell you that the Sigma 70-200 is a good lens
(and then ask yourself, what his own answers to these three questions would be)?
The great problem is that the Internet in particular can be a dangerous tool for spreading (mis)information. No doubt usually comments are made with the best of intentions, occasionally because people just want to sound like they know what they're talking about when really they don't. My bottom line is, think before you decide to comment on a lens or whatever.
BTW, there are people who have posted in this thread who I have no doubt *do* know what they're talking about, so please don't think I'm singling anyone out; I'm not here to identify people, just to serve a general warning about what you contribute, as well as what you believe.