My employer kindly got hold of one of these for me.
As it slides up and down with minimal fuss, I can alternate between sitting and standing whenever I want a change.
The downside is that the keyboard tray only supports 1.4kg - more than enough for most users, but it means the heavier keyboards in my collection just aren't suitable. The tray is a little on the narrow side too, compact and tenkeyless keyboards really come into their own on one of these.
I've splashed out on an anti-fatique mat too. Well worth the money.