I've tried the Whistle 3 and the Wanda's GPS Dog Tracker blows it out of the water, it has so many features and the customer service department is super helpful.
It lets you set-up Virtual Fences and when your dog steps outside the Virtual Fence it Alerts you either by Phone, Email, or Browser whatever you pick. I have one setup really small so that as soon as she leaves the house I know she has left the house, and I have a second larger one which is what I consider her safe space.
The compass that leads you to your dog is awesome and the light you can turn on at night is also super helpful.
I highly suggest the premium service as it lets you export where you have walked your dog and share it with friends and also tracks top speed, along with many other features.
The app works great on my Droid phone and I just found they have a second app that I haven't even tried yet.
Anyway I highly highly recommend this tracker.