It was successful if only because of the timing. Around the time he did that, or maybe it was when he released the first draft for free, it was announced that he would be finishing the Wheel of Time. This led thousands of Wheel of Time fans, who had never heard of him, to descend upon his website. I know several of these people, and while they liked the sample chapters and whatnot, it was being able to read Warbreaker that sealed things for them. It was mostly through that book that they moved to his other books (if they liked his writing) or turned their noses up at it (if they didn't like it).
If he had not had as much exposure... I don't think it would have worked out so well. It would have been better to release the book first, then start releasing the free drafts - as it was, I had to wait months to buy the book after I finished reading the free, unedited version, and I almost forgot about it. The release of a free sample should be as close to the release of the full product as possible. That's why when they have free samples of something at the grocery store, it's not of the stuff that won't be on the shelves until next year.
Of course, the question is was releasing it free a good or bad idea. It was good, clearly. But the timing was bad, and it only worked due to a very specific circumstance not many authors are going to have.