I've had a GeekDesk for a long time now... [close to 10 years?].

I love it; and move sporadically between standing and sitting. Standing is better for conference calls.

Writing prose, emails, books, and blog posts are better done sitting down.

In terms of writing code, I could go either way.

Often switching between standing and sitting is a really good way to fix a problem I'm having focusing.

That said, I go through spurts where I stand a lot and spurts where I sit a lot; there is no real rhyme or reason, I'm not as religious about swapping every hour.

It surprising how natural it feels to stand and work at the desk, and once I get in a zone it doesn't matter if I'm down or up.