I have an Aeron for my office, but I recommend getting one without armrests. But frankly, i'd also like a height adjustable drummer's stool. I have too many instruments at odd heights.

For me, almost everything is armless (I play guitar ), but I do have a small selection that suits most needs.

I like bar stools for the height, and foot rests. I have 2 wooden (fixed) stools, and 2 soft/padded stools.
I also have an adjustable drum stool that is incredibly comfortable. I usually use this for Pedal Steel and Dobro.

For mixing, I have a padded office chair with arms, but is height adjustable. I have to respectfully disagree with bitflipper on this one. I like to have the wheels and height adjustment so I can hear things from many different perspectives without getting up. Maybe I'm just lazy!

I agree with everything everybody has written.

BF is correct if the sweet spot is identifiable.

My room is too small, too varied, and too annoying to not have a wheeled chair.

No arms?  Great idea. Wish I had a chair that could become armless.

Why are the builders of these things so stupid, as to not provide such flexibility?

Buss seats? Airplane seats? IMAX seats?   Who fits any of these?

Who are the idiots who design them?

What sorta brain damage occured to the packleds who buy them?

I've got a Herman Miller Aeron and a Herman Miller Equa - both are excellent chairs.  The nice thing about the Aeron is that the arms swivel and adjust in height.  You can put them down very low so they're almost totally out of the way.  You can also lock in the position anywhere you want to prevent rocking. 

The big thing now in offices is adjustable height workstations.  Staying in one position for hours at a time is not very good for the body - the back, blood circulation, etc.  I haven't seen this in any music studios yet, but some of these adjustable height workstations could easily handle some close monitors so that you could maintain speaker to ear position.  Someday I'm going to install one of these in my house so that if I'm recording guitar, I can stand up instead of sitting in a chair.

I sometimes use a kneeling chair.  In case you're wondering what a kneeling chair is, it looks something like this:

Otherwise I use a standard office chair, one without armrests.

As far as arms go, I diligently searched high and low for an "office chair" with removable arms.
So, the right arm is gone and the left remains. It's all good for guitaring..

Need a vibrating cushion which they sell (all seem to have heat also if needed). Get a massage and circulating going.

Staying in one position for hours at a time is not very good for the body...

True.  Here's a before & after example:

I spend more of my time at my writing computer - like right now.

I find a short stool works well.  I'd love an aeron, but the stool works well.  Keeps your back straight.  I don't sit for hours at a time - it is very easy to get up for coffee etc., but it is pretty simple to throw a cusion on it for buttache.

@

Just picked up a vibrating massaging chair cushion. 3 zones, thigh, lower back, uppper back, all or none or one or two or three, and heat if needed. Works great and got it at a drug store in the local area, not expensive either.