From the article:
Taking into account that we tested it against another 3G phone with a T-Mobile SIM in it, we believe that it's not a network factor, it's the G1's browser and processor being able to render pages much faster.
I love my G1. It feels very fast. When 3g is up, it's almost as fast as a fat client on broadband. I'm glad someone did some tests.
T-Mobile's 3g network is still in the rollout phase, I've heard that most of it will be up (at least here in Cali) by the beginning of the year. That was from T-Mobile salesman, so some salt should be taken with that information.
The phone is really why I bought it, but I must give props to T-Mobile for not making people pay through the nose for the service. AT&T seems to want to fleece the iPhone buyers. Thanks to T-Mobile for at least trying to sell their service at a more decent price.