Ok, some classic FUD that needs debunking.
(a) Is the only real issue here. If you're shooting at f/2.8 or otherwise shooting for scenes where bokeh will need to be considered then you have to be mindful of the potential impact of onion bokeh.
(b) Just about every lens has sharpness falloff towards the edge, including a lot of Canon's "L" glass. If you look at any MTF graph, you'll see this and similarly it will show up in pictures. In this area, Canon's wide angle "L" zooms are atrocious.
(c) If IS is pointless then why does Canon have it on the 24-105, 17-85, etc? For some people that have trouble holding the camera/lens still, IS is a life saver. Lots of people wanted Canon to come out with an IS version of their 24-70, so it is safe to assume that lots of people see benefit in it being there despite it not "slowing the scene down".
(d) nobody has yet come forward with measurements of AF speed, it's all "feeling" based and we're talking differences in 10ths of a second or less. The only people for whom this will be a problem will be the Internet photography experts that shoot from the comfort of their chair at home.
(e) It has yet to be established that there is any significant variation in lens quality from Tamron regarding this lens. Regardless, there is plenty of talk about of people receiving "soft" Canon lenses too. In both instances it is likely a problem solved with AFMA.
No FUD because as I wrote I'll probably buy the Tamron myself for being a lot of "bang for the buck". But I'm sorry to have to correct you...
a) I disagree: the "onion" boekh is *not* an issue because it needs specific conditions to appear, so for many scenes you'll never see it.
b) Just because there are other Canon lenses that show a sharpness falloff, it doesn't mean it can be taken out of consideration - because the alternatives to the Tamron 24-70 are Canon's new 24-70ii or primes, but of course both at a higher price.
c) The 24-105 has IS because it only has f4 max aperture, and the 17-85 and such are consumer lenses that wouldn't sell w/o IS and are dual-used for shooting still scenes, too. For me, the 24-70 *f2.8* is an event/wedding/docu lens, and since you need about 1/100s min shutter speed for shooting people anyway IS is not needed, esp. if it needs a little time to swing in after af'ing like the Tamron.
d) I'm really happy to debate cons and pros of equipment to make up my mind what to buy, but saying I'm an "Internet photography experts that shoot from the comfort of their chair at home" is just bs and only marks you as one yourself. The Tamron is slower to af esp. at lower light, where even a little makes a difference if you use it for wedding/event w/o preflash or af assist - the photozone review says so, see the "nikonguy" side-by-side comparison, and I tried the lens myself next to a Canon mk1. While the difference might not be large, it may make a difference if you shoot hundreds of pictures in an hour with it like during a wedding or event.
e) I'm getting my information from Amazon et al customer reviews this thread: http://photography-on-the.net/forum/showthread.php?t=1145772&page=36
.. and where else would I get information from? Buy 20 lenses myself? While Canon is not a shining light in the past, even the Internet cannot be wrong at least in the tendency that 3rd party manufacturers have more qc issues. But then it has still to be established scientifically that smoking is bad for your health...