Not too early to call him a dissapointment or to suggest he shouldn't have been picked where he was. But to me bust connotes having no game, a guy who shouldn't be in a teams rotation (so by that standard a guy like Shawn Bradley who was clearly poor value for his draft slot, doesn't constitute a bust). At this point I would say he could still have value to a team, at some point down the line anyway.
But his play in the NBA thus far has him on the path to bust-dom. He really needs to get his jumper going if he wants to be a contributor at this level as his shooting percentages have been terrible, as his ability to draw fouls (his only notable offensive skill last year, as his passing, like his shooting was awful) will be negated if teams feel confident sagging off him.
So I would say on balance that he's presently on a bad track, but he could yet add some value. Here's a list of players who had a worse first year by two advanced stat measures. You'll note that whilst most were indeed useless a few became all-stars including Glen Rice, Deron Williams, The Van Arsdale twins, Norm Van Lier, Richie Guerin, Jeff Hornacek, Kiki Vandeweghe, Bill Bradley, Jeff Malone and Curtis Rowe, plus other players with value like White Chocolate, Tom Boerwinkle, scorers like Andrea Bargnani and Dana Barros.
Looking at those above it would seem his best chance to become useful is as a scorer if he can up his percentages.
A cause for optimism is that %s are the stats that fluctuate most and many rookies shoot unimpressive percentages. Still, Williams has a signifcant mountain to climb if he wants to be more than someone at the very edge of the rotation.