I'd hardly call it a failure overall. The FX5800 was a pretty bad slip-up, and they didn't really have a good mid-range solution for the price, but the FX5200 and FX5900 were very good for their price-points. Sure, the FX5200 is slow, but it's still much better than it's predacessor, the GF4mx, and it does very well against ATI's low-range budget card (the 9000/9200). I do give the FX5200 credit for actually offering a high-end feature on an extremely low-priced card, even if it was very slow at it (at least it can run tech demos to show off the prettyness of it
). The FX5900 corrected many of the problems people had with the FX5800, and it's quite fast as well. It still falls behind in DX9 stuff, but it isn't THAT big of a gap (the det50 drivers definatly helped it a lot), and that won't really be a huge factor for some time. For a while there the FX5900 was a better buy than the 9800 cards due to the huge drop in price that happened in late 2003, when you could get them for over $50 less.
The FX series certainly could've been better, but it was far from a failure.