The Sidekick 4G supports T-Mobile’s new , meaning that the handset is capable of reaching the 21 mbps of (theoretical) throughput speed that the T-Mobile network is built to deliver. This does not mean that the Sidekick 4G can actually hit 21-mbps download speeds, but rather that it contains a radio and chipset capable of faster speeds than earlier T-Mobile 4G phones achieved. We using the Samsung in a few cities, including San Francisco, and were pleased with the solid 4G-like speeds. The Galaxy averaged 3.38 megabits per second for downloads and 1.13 mbps for uploads. It couldn’t hold a candle to Verizon’s blazing speeds on its (18.30 mbps and 7.39 mbps), but it outperformed both AT&T’s HSPA+ network and Sprint’s WiMAX network.

So I have to give T-Mobile the benefit of the doubt when I report the rather dismal speeds I achieved with the Sidekick 4G. The South Park neighborhood in San Francisco, where PCWorld is located, has never been a very strong area for T-Mobile. Using the FCC-approved Ookla Speedtest app, the Sidekick 4G averaged 0.91 mbps download speeds and 1.26 mbps upload speeds.

The Sidekick is powered by the same 1GHz Hummingbird processor you’ll find in Samsung’s Galaxy line of phones, like the Vibrant or Galaxy 4G, also on T-Mobile. The browser handled scrolling and loading pages fluidly and native video played back smoothly.

In terms of call quality, the Sidekick sounded pretty good. Callers sounded loud and clear over T-Mobile’s network with no static or hiss. Like many T-Mobile phones, the Sidekick can also make calls over Wi-Fi, which is useful when reception is poor.