Sounds pretty good although it doesn't match the 24-70 II (as expected from the MTF charts), at least not at f/4 (although it's very close wide open to wide open). I'd really be curious how it does f/8 though or even f/5.6 too. It's hard to say much more without knowing that. This sort of lens, in particular, may be used stopped down a lot.
Better than the 24-105 IS (or 24-70 I if you don't need f/2.
though. I suppose it is possible that the 24-105 catches up as you stop down and then the 24-70 IS would not look so good though. OTOH if the 24-70 IS becomes like the 24-70 II stopped down then it would be a much better lens than the 24-105 by miles.
If the Tamron didn't exist I guess it might make sense at the price it was launched at, although it really depends how well it compares to the 24-70 II stopped down. Considering the Tamron though it seems a bit pricey (although, again, it's hard to say without seeing how it does stopped down, maybe it does better for 24mm landscapes?). It only tested a tiny bit better across the range and lacks f/2.8, but again what about stopped down? We only get to see the 24-70 II wide open vs Tamron stopped down in this test.
It appears to deserve to cost more than the 24-105 IS but the Tamron does make the price seem a little rough, depending (on whether it is a trade of for getting access to f/2.8 vs. better stopped down landscape performance or something).
Of course large scale contrast and AF and such remain to be seen.
And the stopped down performance needs to be tested. Does it perform like the 24-105 or the 24-70 II?