I have eggshell on all my walls, with high gloss white trim.

Eliza, I was hoping you would get some more answers. I'm certainly not an expert in this area.

I would say it depends on the quality of your paint. BM Aura has a new matte finish for kitchens and baths, so if you're after a velvety flat look, you can still get it even in greasy, humid areas. In general, I use eggshell for living areas and bedrooms and pearl or satin in kitchens and baths with satin or semigloss for all trim. But, I live in an older home with some visible drywall seams, nail pops, and miscellaneous "wall boogers". If you have perfect walls and trim, you can get away with more sheen if you want. Flat is good if you know you're going to have to touch up paint frequently or for ceilings.

So, is satin like a step shinier than eggshell? What's the difference?

BM uses 'eggshell' to designate what you usually think of satin. We use satin/eggshell on all our walls, and semi-gloss on all the trim.

Eggshell walls, flat ceilings and semi gloss trim.

We have satin walls and ceilings and semi gloss trim (new build). We originally did satin in the bathrooms, but had a horrible experience with the paint peeling off. We repainted the bathrooms in semi gloss and it's been fine since then. Not 100% sure that the paint was the problem.

Most of our walls are eggshell (which looks ever so slightly more shiny/lively than flat). Eggshell is more scrubbable than flat. We have a 12 by 13 foot den with red SEMI GLOSS walls. I am surprized how much I liked the semi-gloss. The light bounces off the wall and makes the small room look lively. Here is pic of the room in an earlier incarnation (before the next wave of my son's instruments ...)

I used satin thru out my house and high gloss on the trim. I love the satin because it doesn't really have a shine to it but clean up is very easy

Eggshell in kitchen, living, dining, and bedrooms. Staircase and baths in satin because of little children who always love to touch the walls...and semi-gloss on all trim. I've never had any problems with any of it anywhere.

Thanks to you all-- you've been much help! Thanks for the pic, deltabreeze... that wall color looks like my son's wall color. Beautiful! I did his in satin and it has just enough sheen for me.