I have an adjustable office chair, it is a high quality brand with dense padding and many adjustments. I have to sit high to reach my machine which rests on a counter-top bought at a yard sale, so hubby is building me a foot platform tonight. Otherwise, I'll get back spasms and leg cramps from hunching over, and not being able to rest my feet on the floor.
IMHO it is all worth it. It is hard to concentrate on your sewing if your butt/back/legs, etc. hurt too much to concentrate! I also keep my ironing board across the room so I am constantly up and down.