If you define the success of a war as having met the principal task you set out when you went in, then this clip shows that Powell would consider it a failure.
In his interview with Rachel Maddow on MSNBC (clip at about 6 minutes, 30 seconds in at
http://www.msnbc.msn.com/id/26315908/#30002514 ), he says:
Our principal task--even though it's been, I think, missed in recent reporting--we went in there and after we got rid of the Taliban government because they wouldn't turn over al Queda, we then focussed on going after al Queda and the Taliban.
<snip - talks about the need for the subsequent reconstruction there, and the efforts they made, then continues...>
But we did not eliminate al Queda, we did not eliminate the Taliban. Could we have, if we had more forces? That will be discussed and debated for years to come.
(Note that the official transcript (
http://www.msnbc.msn.com/id/30002070/ ) messes up the punctuation. If you read the transcript, it sounds like he said our principal task was the reconstruction of the country. But if you hear him speak the words in the clip, it is clear that the "principal task" phrase refers to the words that came afterward, not the words that came before. Punctuation can change everything...)