It's not a bad game, but if you are looking for an easy 200/200 achievement points, look elsewhere.
I'd like to see it updated for WP8.
Oh, as far as GPS, yes it works, however, if your phone has no cellular connection, like a dev device, it doesn't seem to get a location... I upgraded my 900 to a 920, found that it doesn't show up on WP8 marketplace and my sim-less 900 won't pick up on my location now.
It doesn't take a lot grinding to go through the story mode (5 missions if memory serves me correctly)... Once you complete story mode, a mission mode pops up. There are upwards of 25 missions, each is some variant or combination of the story levels... Some you have to make it through without getting hit, some are more difficult, etc..