Well, I do use a case but my reception doesn't seem to change with or without it.
It's been an issue since I was on stock rom/radio it seems, although I was only on that for about a day? I was running superclean before this, it was packed with this radio, and it hasn't gotten any worse or better, it just seems bad overall. My girlfriend has the old Droid Eris, it seems to have MUCH better reception. Also, we had to use her GPS when travelling because mine wouldn't lock on, put it in the window, let it try for an hour before giving up. That's the only downfall I see with the Fascinate so far.