At release, the 770 was a good value in the $400 price point. It offered previously $500 performance for a $400 price.
The 280x offers $300 performance for a $300 price point. It isn't a bad card, but it doesn't have the value that should be associated with a rebadge. It should be cheaper than the card it is rebranding. Another issue is the 7970 is a good value already, so the 280X has to be a better value (which it isn't).
One quick point - The 300$ 7970GE only happened within the past two weeks, it is a recent development to clear out inventory for the newer R200 SKUs. Prior to the GPU14 event, I regularly saw 7970GEs for their 400$+ pricepoint continually on newegg *and amazon. Only recently did *all* of them go down to 300$, it is simply to clear inventory. Basically, the 7970GE is on par with the 280X pricepoint but I would imagine the 7970GE is an EOL product.
In terms of performance you get for the buck, the 280X is just better than the GTX 770, period. Hopefully nvidia will lower the 770 price a tad to better solidify the value proposition - when the 770 was initially released, it was a great value proposition. Now? Not so much. If it were adjusted down to 350$, it becomes a great value once again, IMHO. It comes with a free game and offers a heck of a lot of performance - which would be fantastic for a 350$ pricepoint. At a 100$ price premium over the 280X, I think that is a bit much.
The bottom line is the performance you get for the dollar, and right now the 280X is just better than the 770. I would like that to change, I want the 770 to be better positioned against it. Don't you? The main qualm I have with the 280X is that it isn't exactly a performance progression, but that isn't a big deal considering most 280X cards are clocked higher than 7970GE speeds anyway and have an attractive price. The other qualm is whether the 280X comes with Never Settle. If they do not, I would chalk that up as another negative against it - but not in terms of value which remains high.