Free gear. Last year was the Nexus Q, Nexus 7, and Galaxy Nexus.

The talks are good too, but the appeal has definitely been the gear, which is unfortunate.

Edit: to add, interacting with the devs is really nice. I went 2 years ago and was able to talk with the GWT team a bit after one of the talks. As well, if you want to network, there is definitely lots of opportunity for it, but it is not something I've cared about.