I've not got a 4S, so am doing the reminders by typing them in not through Siri... but I've still got a problem with it!
My problem with it is when I tap "Current location". It very quickly gets a location, and then saves that. Unfortunately it doesn't spend the time waiting to use GPS or anything, it just uses the highly inaccurate cellular triangulation location. e.g. it locates to a few nearby roads. I'd much rather that the reminders app waited a while to get the accurate location!
If I open maps, give it a few seconds to get the location by GPS, then go into the reminders app and set the reminder it then gets an accurate location. e.g. the exact house I'm in.