They show negative health effects from prolonged sitting. The "prolonged" part is important.
Standing desks are fine if you are using one because you like it better than sitting, and are aware that there are health problems associated with too much standing.
For those considering standing just because of the health problems of prolonged sitting a much simpler way to avoid those problems without exchanging them for the different set of health problems from prolonged standing is to set a timer to remind you to take a short break every 20-30 minutes. During that break get up and move around for 2 or 3 minutes--you don't have to do anything vigorous for this, just taking a walk around the office will do [1].
It's interesting to note that the common time interval used by people using the Pomodoro time management system [2], 25 minutes, fits in very well with the above recommendation.
It is one of the reasons I'm looking to work while walking in nature. It works for 50% of my work load currently (meetings and project discussions / management tasks) but for coding I still need a computer. I am getting there though; my ideas for mobile coding, mobile internet and voice recog are getting better and faster. The biggest challenge is how to make overviews; there is no way to browse through code easily on a small screen so I make overviews in different ways and that part I have not managed to tackle comfortably at all.
I cannot recommend this enough for anyone sitting in front of computers > 4 hours / day.
The chair is also very important (does it help you to have a correct sitting position) and should be adjustable to have a 90 degree leg bend too. And the screen should be in front of the eyes while sitting and standing (standard feet for many computer screens raise them way too little from the 90cm desk), so you may need an extra arm.
While standing, I try to take my shoes off as shoes (even if by 1cm) usually alter the natural position of the feet on the floor and that goes up to the back too. I've also seen people stand on soft playground mats for extra comfort.
The downside of all this is the prices (good office chairs starting on 800 euro and same or more for standing desks). Hopefully your employer can provide it...
Now looking to get a setup with an overhead bar so that I can do some some additional arm exercises (pull ups, etc.). I already keep two free weights within easy reach.