I use my GV number for craigslist and usually place calls via the google voice webapp. Works great. It winds up routing placed calls through a proxy number, but your GV number shows up in the caller ID of the person you're calling. It still uses your wireless minutes (e.g. not VoIP), but minutes aren't a problem for me.
Integration with the address book would be nice via the web app, but most people in my address book I call with my real number anyway. I think there's a native GV app for JB iPhones but I don't have any experience with it.
