The FM reception in my xB is as good as, and usually better, than the FM reception of friend's non-xB vehicles when adjacent.
The biggest problem I have consistantly seen with reception in an xB is with the folks who have replaced the Scion head unit and failed to re-connect the antenna amplifier power. (The xB does have a built-in antenna amplifier that needs to be powered by the head unit. If it is not powered, the head unit does not get an adequate signal.)
For all you folks with aftermarket head units and poor reception: As I said above, when the new unit was installed the installer probably didn't hook up the power antenna lead from the head unit, and your in-line antenna amplifier is not being powered.
I also have had no reception problems and I'm in BFE (read: Iowa). My "diversity antenna" (that's what they call it, I swear!) has always picked up everything just fine, but I'm listening to XM most of the time anyway so...