I always have these thoughts hit me when I decide to start a new campaign:
Do I spends a few days writing one up and mapping it out?
Do I pick one of the myriad commercially produced settings off my shelf (virtual or real)?
or Do I pick a commercial setting and tweak it to make it something closer to what my vision is?
Invariably I choose the latter option these days.
The reason is simple. I no longer have the time to spend hours and days working on a setting, then even more time working on the adventures and yet more time actually running the sessions. Adult life seems to leave me with adult responsibilities these days. The nerve of it!
All this means is that I need to have most of the setting work done for me. Something like Forgotten Realms doesnt generally work too well from my perspective - there is too much cannon involved to allow it to tweak well, and there's too much history for me to even want to run it accurately (or have a snowballs chance in hell of accomplishing).
In my ACKS campaign I used Blackmarsh by Rob Conley, heavily tweaked but it worked fine (as an aside, ACKS has too much world setting built into it's character classes and too little world setting included to make those classes work properly, but I digress)
For the AD&D / Rappan Athuk campaign, I'm using the setting included in the RA book itself. It's not a huge area, but more than sufficient for now. The great thing is, the details are sparse enough that it's easy to tweak. Win Win! ;)
So, what it your preference?
Homebrewed setting, off the shelf or some mishmash of the two?