I just got my 4S yesterday (thanks AT&T for the early upgrade date!). I've noticed two problems with Siri and I don't know any good methods to troubleshoot.
1) When I hold the phone up to my head from the home screen, Siri starts, beeps and etc. When my wife holds the phone to her head, nothing happens.
2) I've tried to set "geofenced" reminders -- "When I leave here, remind me..." sort of stuff. I've not yet actually gotten the reminder to trigger, even though Siri told me it was set.
I'm sure I'll run into further Siri issues as it becomes a bigger part of my life.
What I'm asking for is not specific answers to the problems listed above (though they would be cool). I'm trying to get a better feel for generalize Siri troubleshooting techniques.