I stopped, unless I know for sure the person is trustworthy. I've had a few test drive and know how to shift very well.
The last person, who has driven and raced manuals for a long time, never blips the throttle when down shifting. He literally slams it in gear and force the synchros to play catch up by several thousand RPM. I turned S-Mode off for him because I was afraid it would interfere with his driving. It drove me nuts to see him go from 6th to 2nd at 40-50 MPH without blipping the throttle. The no lift shifts were terrible because the car would jump nearly 1000 RPM in between shifts, so it's slow.
When I had the 350Z I let this one guy, who told me he is very experienced driving manuals, drive it. First thing he did was kill the car. A few minutes later my car was releasing the burnt clutch smell.
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