I'm currently using this one http://www.ikea.com/us/en/catalog/products/00103102/.

I get sore feet normally from standing too long, and tend to buy cheap shoes, so the mat helps a lot with comfort.

Have you considered in-sole gel pads for your shoes? They aren't too expensive (~$15), and I've personally found them to be very helpful when going for walks, and they work very well for my mom who's feet heels were actually in pain until she started using those (though once she got professionally fitted properly by an expert at a shoestore, she no longer needed the gel pads).

The benefit being the gel pads move with you even when you're not working.

The most comfortable space I've worked in had quick swap desks. Monitors were on heavy duty swing arms and could switch between the seated desk and the standing desk in a few seconds, then a keyboard with USB port for the mouse to minimize cord tangle and allow quick switching. You generally started off standing in the morning, sat around lunch, stood for a few hours in the afternoon, and then finished off the day in the chair.

A standing desk does Not replace a normal sitting desk, but rather works with it for best results.

Ideally you're also moving around far more in the office, and not tethering yourself into a small cubical. Get up, move around, get a large table on the other side of the room for papers and such. Change things up, stay active, and pay attention to what your body is telling you. If you're sore, something is wrong and you need to figure out what is causing it.

I get sore feet normally from standing too long, and tend to buy cheap shoes, so the mat helps a lot with comfort.

Have you considered in-sole gel pads for your shoes? They aren't too expensive (~$15), and I've personally found them to be very helpful when going for walks, and they work very well for my mom who's feet heels were actually in pain until she started using those (though once she got professionally fitted properly by an expert at a shoestore, she no longer needed the gel pads).

The benefit being the gel pads move with you even when you're not working.

Omiting that someone has "flat feets" or any other indisposition for standing, Standing, at least permanent one, is realy not it anyway. The fact is that calves and legs cannot unflex the muscle what gives unnatural stress, and it is even more unnatural if you are standing without ability to take a step (infront of a computer) .

More comfortable would be even the escalated idea of Olof, when you would stand on a walker machine- that would for example turn rolling if you lift one feet-smartly.

I myself use a crashed rolling flying chair