In Lakewood, it's common for a Sefardi to daven in an Ashkenazi minyan; but an Ashkenazi would never be caught dead in a Sefardi shul. (This has more to do with logistics than anything else, but still a funny observation.)
I don't think this is only in Lakewood.
In EY I found it more common to find myself in a sefardi (weekday mincha or maariv) minyan (either in a place where anyone can go to the omud and daven their own nusach, or where it was most convenient.
At a time when I had a pinched sciatica nerve, there was a sefardi minyan accross the street from me, so I davened there. I also took my son to daven there a couple of times Friday night for the experience.