Hello Annika, I never trust people claiming things like this.
I have not bought it yet, but I'm quite sure, I'll get another Tamron. I came to the conclusion, that a (rather) wide aperture at 75mm is more important for me than an extra few mm. And yes, I know that there is IS.
Why not get another Tamron 28-75 2.8? It's light, sharp and cheap compared to the other choices. I've had one for years, and it has served me well. I'm only moved to the 24-70L (big and heavy brother of the Tamron) because I needed the weather-sealing for my work.
Oh, I dunno about that, Annika. I bought a Tokina zoom in preference to the equivalent Canon lens for my (rarely used) 30D. The Tokina lens is sharp and much better made than the Canon. It also fits my film EOS bodies where the Canon equivalent is an EF-S model which is said to be incompatible with those cameras. I've been known to use Voigtlander and super-cheap Russian/Ukranian lenses on my Leicas.
I can say that the L Canon glass is superb (24-70 2.8). I haven't used the 24-105 (f/4 is just too slow for me) but if you don't want to pony up for the Canons I have seen some pretty surprising results from the Tamron. At one point I thought about buying it for a beater lens. Skip the 28-whatevers from Canon.
I recently asked this question on a Flickr group (for my 5D) - and got the same kind of reply - 'how can you even think of using anything but the 24-70? Why don't you sell your camera and buy something cheaper so you can afford the 24-70????'