It's as good as the various location technologies (GPS, WiFi, and Cellular) of your device allow. Depending on the conditions, accuracy will vary, but LocationSwap always makes an effort to provide the best reading it can.
In order to conserve power, LocationSwap does not keep the GPS switched on continuously. If someone is moving, the position you see will generally be updated every few minutes, as long as the device can connect to the internet via 3G or WiFi.
If your iOS device does not have a GPS, the accuracy will be lower.
LocationSwap isn't funded by selling your information or displaying adverts, so there is a small upgrade charge which pays for development and operation.
If you restore your new phone from a backup you made from your old phone, LocationSwap will continue to work as before.
If you don't have a backup, you will need to install LocationSwap again. If you have upgraded, you won't be charged a second time.
No. As long as all your devices are using the same iTunes account, you will only be charged for the first one you upgrade.