Does the location reminder use a lot of battery, and what happens to my location?

No, it doesn't use noticeable battery — there's a clever trick behind it. Your iPhone already roughly knows where it is at all times (via cellular and Wi-Fi, not constant GPS). pon uses exactly this built-in „Geofencing“ feature of iOS: you store your stores, and the operating system itself watches over them in the background in a power-saving way and only wakes pon the moment you reach one of your stores. So pon doesn't run the whole time and never keeps GPS on permanently.

And on privacy: your location is processed solely on your device and never sent to us — we don't want to know where you are. That's exactly how pon taps your iPhone's potential without you giving anything up for it. Give the location reminder a try; if you don't need it, you can turn it off again with a single tap.