I've been using the Flytrex Live 2G version for about 3 months now. I love it, and it's pretty much the only reliable way to track your bird in real-time on Google Maps when doing long distance flights (>2500 feet). After you power-up the Flytrex and see the flashing green LED to know it's transmitting data to the cell towers --- you click on "Live Flight" channel in your Flytrex Profile and you will see a moving red dot on Google Maps (satellite view) that represents the position of your bird as you fly. None of the "dial-up" GPS trackers can do this.
HOWEVER --- get the 3G version if you live in the USA. I always have data transmission problems with my unit even when I fly in an area where T-Mobile coverage maps show solid 2G reception. I've been hearing the 3G version has mostly solved this problem (mainly because of excellent widespread 3G coverage in the U.S.) --- so I'm gonna buy it soon and compare it to flying areas where I have known problems with the 2G version.
I think the best deal on the SIM card data plan is from H2O Wireless. The Flytrex uses VERY little data per flight.