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That's what I had heard too, but I can't remember where. In practice, it kept location services on the whole time the reminder was set, which took my battery from 100% to 19% in about 4 hours. I'm really disappointed, because I was really looking forward to using this feature.

I have experienced no additional battery wear using geofence reminder, which really surprised me.
 
I tried this for the first time today.

"Siri, remind me when I get to work to check my 3G speed."

It pinged just as I shut off the engine in the car park. Beautiful. Also it had no obvious impact on battery, which I didn't expect since the GPS icon is on when using a geofence.
 
I've used a location-based reminder twice since getting the 4S and both times it worked perfectly. Once when pulling in to the parking lot at work and once pulling in to my garage at home.
 
I set one up for testing purposes and with a very busy day, forgot I had done it. When I pulled out of the parking lot, the message popped on screen. I thought- now THAT'S pretty cool! It remembers even when I don't remember there was anything to remember!:)
I've used it a handful of times since and it has worked every time within a very short distance of the location I've set.
 
i set mine to grab stuff before i left the house cuz i always forget and as soon as i backed car out of the driveway it went off. i would have drove 15-20 miles before i would have remembered. i can't wait for my 4s next month so i can just tell siri and then i won't have to type
 
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