It's a mixed bag.
-Robert Jordan got too bogged down with plot after book 5. The books were still entertaining, but they were sloooooooow. Brandon Sanderson reads much faster.
-The female characters have more distinct voices under Sanderson. Jordan's female characters were all variations on the same theme, and were nearly indistinguishable from each other absent individual quirks (Nyneave pulling her hair, Elayne's cursing, etc.).
-Some of the characters are a bit off, Mat and Perrin in particular. I like Sanderson's take on Mat, but it's a much different version of the character than Jordan established. The same is true for Perrin to a lesser degree. On the positive side, Jordan's characterization could be a bit flat at times, while Sanderson gives each character a distinct voice and personality.
-Sanderson just doesn't know the material as well as Jordan. There are more mistakes and continuity errors that might drive a hardcore fan nuts.
-Speaking of continuity, Jordan usually kept events moving chronologically; character POVs from chapter to chapter happened within the same frame of time. The problem is that Jordan deviated from this in the last couple of books with respect to Perrin's storyline, which ended up a month or two behind the rest of the storylines. This was a big problem for Sanderson, who had to catch Perrin up while moving the other characters toward their own endgames. The result is that one character shows up in two different places in two consecutive chapters, becuse those chapters do not happen chronologically. It's not really Sanderson's fault, but it is a bit jarring.
On the whole, I've enjoyed Sanderson's work. I like the more dynamic character elements he has brought to the story, and while some of the world-building (Jordan's specialty) has suffered, that isn't quite so crucial to the story this late in the game.