At the same time, at the same place, with the same GSM provider: a simple comparison between the reception of a $30 Nokia and a $600 iPhone 3GS. This should wipe the excuses from Apple that it is all about "the formula we use to calculate how many bars of signal strength to display", right off the table.
It is black and white: either you have reception or you don't. Either you can make a call, or you can't.
In my apartment in Rome, where GSM reception is rather marginal, I had to buy a $30 Nokia with a spare mobile number, so people could actually call me.