Yes, it is certainly healthy to work standing up. Obviously not all day long… because standing (in a static position) for too long is not ideal either. A few of the benefits of standing up are that your blood circulates better, your body absorbs more oxygen and your back muscles get stronger. You are also more alert, less sleepy, and you can concentrate better.
The trick is to alternate regularly between sitting and standing at work. You can stimulate this with the right office furnishings, such as a sit/stand desk by Sedus or Wilkhahn and a good office chair that encourages active sitting.