I've never really had separation between home and work. I started my first business in college and have worked for myself ever since. I've had offices/warehouse related to some of the ecommerce stuff I've done, but even then I'd work from home a lot. My brother is a developer at an agency that I sometimes consult for. They keep a desk open for me there too so if I want to work around people I'll go in there.

So there isn't much physical separation of home and work and there is really no mental separation either since I own my own business.

For the most part I'm able to turn it off when I need/want to. If I'm not feeling productive or just want to do something else I can set the work down and pick up a guitar, go work out, ride a motorcycle, go see a movie, etc. and not think about the work until I get back to it. If I couldn't do that I'd probably prefer more separation of home and work.