Just found this nicely in-depth overview of the Steam Controller from the most trusted of sources at CES 2014.
The consensus between them is that it's interesting and does some things well, but gives a bad first impression, has a really big learning curve, and the set up for certain actions is frustrating. To sort of sum it up, Will says:
"The aiming's better than a gamepad. The movement's twitchy."
Will also makes the telling point that Valve seemed to only be demoing the contoller with FPSs, not mouse required games like Civ V or Starcraft 2, the games that ostensibly this controller will excel at.
To get in before the inevitable Valve bashing in the comments, I'd like to start the conversation off by saying, even if Steam Machines are a total failure and this controller sucks and disappears, I'm sure that the experimental things it does do well will influence controllers of the future far more than the new XOne and PS4 iterations. Valve is willing to take risks. And even if they're stupid risks that won't pan out, I think it's pretty damn exciting for the industry as a whole.