This has been an interesting thread to follow.
I think that third party lenses (eg Tamron, Sigma, Tokina and others) have their place, particularly when the prices are much lower than similar lenses
Some time ago, I was looking at replacing my Canon 100-300mm USM which I had for some years. For replacements, I was thinking of the Canon 70-300mm (micro)USM IS, and was also considering the Tamron 70-300mm VC USD which I had seen some good reviews of. I didn't want the Canon 100-400mm L USM IS (because of its weight / size, older IS, push-pull action and design). So after reading various reviews (professionals and 'in the field user reviews') - as well as knowing how good the Canon 70-300mm was... I basically thought I would go with the Tamron 70-300mm VC USD, and even bought a good quality 62mm filter for it - that came up at a good price. Neither the Canon 70-300mm or Tamron 70-300mm had quite the quality I wanted, but I NEEDED both IS (or VC) AND more contrast & sharpness than my Canon 100-300mm could offer at 300mm, as well as being sharper wide open. So I was on the edge of this purchase 'with a sigh'.
Then the Canon 70-300mm L USM IS was announced. While it was a higher price... I immediately was interested, but thought... hmmmm maybe not - it's a big white L (whereas I wanted and needed something portable). However a few weeks later - when I had tried the Tamron 70-300mm VC USD in store, I asked to see the Canon 70-300mm L. I was immediately impressed how portable it was - and yes, it fit attached to my 7D in my existing LowePro shoulder bag. The salesperson offered me a good price, and would throw in a quality Hoya CPL filter at a discounted price too.
After going home and checking some more early user reviews as well as pro and site reviews, I decided I would purchase it... as it really met all my needs (in terms of IQ, USM, IS). I have not looked back. A few years ago I was looking at purchasing a telezoom - and was looking at a few Sigmas (eg 80-400mm, 50-500mm, 135-400mm, etc)- but none of the lenses, including the more recently introduced 'OS' versions had as good image quality AND HSM focus and OS, in a truly portable and 'shoot all day without any strain' size / weight. Also, generally I found some of these lenses did not have the best bokeh. Some of them did 'some things' well - but not 1 lens met exactly what I was looking for.
When I was looking to purchase an ultra-wide zoom, I ended up looking at the Sigma 10-20mm HSM f4-5.6 as well as the Canon 10-22mm USM f3.5-5.6. I ended up getting the Sigma 10-20mm mainly because the image quality was basically identical on both lenses (both are capable of producing great, sharp, contrasty photos) - whereas the Sigma's price was almost half that of the Canon's... plus the build quality of the Sigma is a bit better, plus it came with a lens hood included (and the size / shape of the hood much better than the Canon version). However, there is one 'niggly little thing' with my Sigma lens (apart from the 'different zoom ring direction' to Canon lenses)- and that is that the Autofocus is not always consistent, nor 100% accurate. Thankfully the way that I use my ultrawide lens (for 90% of the time it's for landscapes) - I just use manual focus, and because of the extremely wide depth of field, all the shot is in focus (and sharp!) The remaining 10% of my use of this lens is a mix of architecture (still use MF) and some 'special effect' photos - which occasionally benefit from AF. Still, for the very few times I use it on AF, it's not an issue. (PS - and the Sigma 'yellow cast' is not really a noticeable issue on my lens, if I need to I adjust in post processing).
Then when I was looking to purchase a macro (I wanted something around 90mm - 150mm) - I didn't find a lens that had the focus I needed (true USM / inner focussing) as well as the optical quality and size. Whereas there are many very good, ultra sharp macro lenses out there, also from third parties- I ended up buying the Canon 100mm (non L). It has the characteristics that I needed, without a compromise on IQ.
Ok, the point of all the above is that I do think there are many great third party lenses... and indeed there are some lenses made which the original camera body manufacturers do not have an exact equivalent to (eg the Sigma 120-300mm f2.8, or some ultrawide primes, or the Tamron 60mm f2 macro). However where there are very similar or equivalent lenses, generally speaking - the original manufacturers lenses seem to have a bit of an edge. Note I say 'generally'. Just as in this case, I think the Tamron 24-70 will have 'great IQ' (and it has the bonus of VC) - but - it won't have the absolute 'stunning IQ' of the new Canon 24-70mm USM, plus I expect the Canon will probably have superior bokeh.. I can understand why a lot of people will be getting the Tamron 24-70mm (particularly for the price) - yet most professionals will probably buy the Canon 24-70mm mk II. There is a place for both.
Happy shooting everyone!! Competition IS good.