The deal is on a Pay as you go handset, however CPW phones are generally free of any sim-lock, so you could leverage that old PAYG SIM to get some cheap Windows Phone action.
There are some compromises to make to get the phone down to a price though. The screen is a Truecolour LCD rather than the ClearBlack one on the Lumia 620. You'll also lose out on the front facing camera, so video calling with Skype will be out of the question. NFC is gone too.
Still at half the price of the 620 that's not a great deal to give up, especially when you consider that you'll be getting a 4" screen, a full 8GB of storage and an SD card slot. And the class-leading smoothness and slick operation that Windows Phone has to offer.
At this price it really is a steal.