I bought a sit-stand frame from Ergotron[0] which I fitted my old desk surface onto which saved me quite a bit of money.
I don't like cranks or electronic motors. My criteria for a sit/stand desk was that it must be fast and practically effortless to switch between the two positions. For that you cannot beat a lift-system like the one in the Ergotron.
[0] http://www.ergotron.com/ProductsDetails/tabid/65/PRDID/757/l...