I find the foam seat too soft and that my bony ass ends up supported primarily by whatever the bottom of the seat is made of. This isn't terrible, but it isn't great. I weigh about 80kb/175lbs, for whatever that's worth.
I also find that the curve on the back protrudes too far forward at the lower back. If I spend a couple hours in the chair, my back inevitably ends up sore. This is the major problem I have with the chair.
I will say that the chair is sound without the arm rests, so if you aren't an arm rest person (I'm not) you don't need to attach them.
As with many things in life YMMV. Unfortunately, this is a hard one to try out long enough without buying to know whether or not this chair is comfy for you.
ETA: I'm about 182cm/6'1" if anybody is curious. I'm mostly tall in the legs.
I'm sorry, it's such a funny professional deformation.
I'm 92 kg and 188cm for comparison.
RH is one example. I'm particularly fond of Opsvik's designs, which are sold by Varier and Håg. I think active seating is the way to go, and both the Gravity Balans and the Capisco are masterpieces of this approach.
I don't like everything about the chair, though. The hand rests prevent you from having the keyboard up close in most settings (if you like that), but the worst thing for me is the slight slack that the base has had from the beginning (the whole chair tilts a bit over the base). It almost made me return the chair in the first place, but as the sample model at the local IKEA had the same feature I accepted this as a necessary evil in the price point.
I probably won't buy another one, though – I much prefer firm chairs.
I just recently got a new chair, but I can still say only good things about IKEA Markus.
One of the wheels broke in transit so I went round to the local branch and they gave me a new set for free!