Our dog is a working labrador, who isn't very well trained at the moment. I was taking him for a walk and he was on the lead as the next field had some horses in it. We put up a hare and I just let go of the lead (better than a dislocated arm). He charged through the hedge in chasing the hare. I wasn't too worried as the next field was ploughed and I thought I would be able to see him and whistle him back. However, after clambering through the barbed wire and blackthorn, he was nowhere to be seen and didn't respond to the whistle. It's a big field with thick hedges and after half an hour of looking I called out International Rescue in the form of my wife and daughter. Further efforts to find him failed miserably. Eventually, I climbed into a copse in one corner of the field and found him in the middle. He had done a couple of circuits of a tree and his flexi-lead had got wound round it. He was sitting there with a "What took you so long look" and hadn't bothered to bark when he was stuck.
So onto the GPS tracker. It's very small and light and will easily attach to his harness. It works via SMS. Note it only works on 2g networks (ie not Three), although 3g models are available. You need a cheap PAYG SIM. You configure it by sending text messages. Once it' set up you can send it a text and it will reply with a Google maps link to its position. It can also just send you the Lat and Long coordinates and attempt to look up a property address using Google.
Also, it has a motion sensor and you can configure it to send a text if it's moved and then log positions at regular intervals. The model I chose has a mini transformer, so you can power it from a car and use it as an anti theft device.
There are many configurable options to alter its route tracking behavior and motion sleeping. With moderate use, my unit consumed about 40% of its battery in 24hrs. Obviously, this depends on how many test message it sends.
For a much more detailed review, I would suggest that you watch the AndroidAndy review on YouTube, which I'll link to in a comment.