I wasn't quite sure what to expect from The Gathering Storm. I like the Wheel of Time series. I like Brandon Sanderson. However, I tend to be somewhat wary of these "posthumous collaborations," because usually they're only a step or two above fanfic (if that).
In the case of The Gathering Storm, mt worries were unfounded. This is a truly excellent continuation of the series. I think it's actually better than some of the middle of the series novels where Jordan let the plot lag a bit too much. The plot doesn't lag here, and Rand's character actually develops.
My only complaint, is a general complaint with the series: there are too damn many major characters. This means that many of the characters from the past who were interesting seem to either be given less time in the novel (Mat doesn't show up until halfway through) or disappear altogether (Elayne for instance doesn't get anything more than a few mentions). Even so, this is a good book.