Hello Donald,
hmm ... i'm not sure about that. Sorry if the following is hard to be understood ... no native english, no dictionary around ... but i'll give it a try:

On that 105 EX .. if the tube is at full lenght, the front part isn't really fixed. Let me illustrate that:

Side view from the lens (=) with the extended inner barrel (-):
====---- ^
Than the front can be moved up and down as showed by the last ^ .. sigh .. one word if one finds the right one in english .. ;)

I expect probs like that on a zoom-lens .. but not if it's a fixed-focal-lenght.

Secondly, Sigma still uses small screws to attach the single elements. But what good is a screw, if srewed into plastic? Let that lens drop down once and your chances are good, it's broken after that. I constantly find myself handling that lens with a lot more care than the other ones. Those others are glue-ed. Of course that doesn't look as Pro-like as screws do .. but considering that most airplanes are glued too .. it's fine with me .. cheaper and better.

Other things: the AF (though a lot better than on the 50EX) is very slow and loud (not that shrill sound of the 50EX but still noisy) .. and hits the point not all to often. Of course, that's of not to much importance on a macro-lens, but it still is a problem of that lens.

Compatibility is another Sigma-Problem. Tamron uses licensed Canon chips. Sigma builds them by reengineering so that lot's of older sigmas won't work on the 10D / 300D for example. Will this one work on my 10D Mk II i'll buy in some years? The canon certainly will!

Why are there two points to switch from AF to MF?

Last (and least) i've heard stories about the EX-finish breaking away after some usage .. can't verify that for now .. as i do have that lens for just 2 or 3 month.

That's just about mechanical qualities. Optically, i've heard excellent stories and very bad ones. I'm happy with the pics i get and can't afford the other ones so i don't care about them.

To any reader: please keep in mind, that i can just speak of those two lenses .. maybe, the other Sigmas are a lot better!!

Btw: any chance to get that fuji-pic smaller? Makes this thread very hard to read .. sigh

And one last question: anyone using the new Sigma EX 80-400/4,5-5,6 OS CA? May be an alternative to the canon 100-400 IS as it's 300 Euro cheaper ..

Best Regards, Tom :)