I don't think Tahiti is a failure for AMD at all, the cards perform really well nonetheless and pull some great numbers. IMO, the biggest issue is with pricing at the moment.

AMD tried to beat nVidia to the punch of having the fastest single GPU, they did for awhile but now it should focus on doing what it did so well with the 5870/5850 & 6950/6970, price their cards lower than nVidia's equivalents. A $299.99/349.99 7950 would be an excellent deal IMO, so would a $399.99 7970. Are these expectations unrealistic? Right now, yes, but in the near future it should be the main aim for AMD.