I started with a makeshift standing desk, then went to a Varidesk, and now I'm on an Ikea Bekant sit/stand desk. To be honest, it only gets lowered for sitting a couple of times a week (at most), as I've found that I just prefer standing most of the time. And when I do sit, it's on a balance ball.
Also, I would recommend trying something makeshift first (like this: http://lifehacker.com/5919778/build-your-own-sturdy-good-looking-standing-desk-for-less-than-25
), then just going straight for something like the Bekant. Varidesk and the like are OK, but leave a lot of the desk unusable when standing, so they're pretty inefficient when it comes to actual workflow. Love my Bekant, though.