Wow, had no idea.... here you can clearly see the relativity of the price.
OK, save for Nikon 24-70 then - I think it is definitely worth it (the infamous zoom grindig should be gone by now with the latest series).
If you are considering Tamron read this and be careful! At least they are known to have a very long warranty
David, why not the new Tamron 24-70/2.8 VC...? I´m sure you will appreciate both the image stabilizer and the f2.8 for playing with the depth of field (which is a strong creative aspect in certain situations the 4.5-5.6 zoom cannot really give you).
p.s. Yes, the Sigma 24-70 is not an example of well-done lens... It is really below average in optical quality - don´t spend on it, sooner or later you would regret it. Based on my experience, if a more expensive thing brings a real quality, the initial high (and rather painful for sure) investment becomes soooo relative in the long-term satisfied use...