Dunno; I haven't actually tried using it yet which is why I said "may want to consider using". Still, it's likely the only thing that works reliably. As previously noted, significant position change notifications rely on cell towers (and perhaps WiFi?) and so might not work reliably in many situations: deep canyons, very remote locations, etc.

I would hope that geofencing is reasonably efficient, though I'd certainly want to test it to make sure before I released an app. I'm sure the Smart Guys at Apple would have thought of all the same power-saving optimizations I've thought of in the last 5 minutes: piggyback geofencing on significant position changes if they're available, if significant position changes aren't reliable use location/speed/heading to determine how often to check GPS, etc. If there's a geofence around New York City, for example, and the device is currently in Los Angeles traveling at 10mph and headed south then you don't need to check GPS very often to see if you've hit the geofence. This sort of optimization doesn't work if you have lots of overlapping geofences near your current position, which is where something like the Reminders app could run into trouble.