Typically have them use the gun, even then with the glass, sometimes they have to get the angle right.
I won't do flat scanners, as the phone has to be turned down, and I definately don't want someone else handling my Droid!
On the self checkout units I've seen, the scanner has worked with it, just again have to find the angle, etc.
I like it!
At worst, if I don't want them to handle it, I let 'em read the numbers off and punch them in, still cool that I don't have to carry all those cards ...