Data plans and performance
We tried the T-Mobile Sonic 4G Mobile HotSpot around the San Francisco Bay Area, and similar to the Rocket 3.0 Laptop Stick modem, it didn't offer anywhere close to 42Mbps download speeds. This doesn't mean the mobile router doesn't offer that speed in your area, however, since mobile cellular data speeds tends to vary a great deal from one place to another.
In the end, on average, we found that the Sonic offered about 3,000Kbps up and 1,200Kbps down, which are not slow speeds but much lower than other mobile hot-spot devices we've seen. The Clear Spot 4G+ Apollo, for example, offered around 9,200Kbps up and 3,300Kbps down when we tested it.
Nonetheless, the Sonic's data speeds are fast enough for any Internet applications. In fact it could blow through 10GB of data in just about 4 to 5 hours. That 10GB, by the way, is also the data cap for the top plan that T-Mobile includes with the Sonic, which costs around $80/month. There are also two other data plans with monthly caps of 5GB and 2GB that cost $50 and $40, respectively.
While limited data plans are standard for most carriers, it's disappointing that the Sonic has no option for an unlimited data plan. For this reason, maybe it's a good thing that the Sonic isn't as fast as T-Mobile claims, or else you would go trough the allowance in a heartbeat.
The good news is, in case you indeed use up the monthly data amount, you can still use the device without having to pay more. The only catch is that it will work much slower for the rest of the month. In a way, the Sonic offers a somewhat semi-unlimited data plan.
The T-Mobile Sonic 4G Mobile HotSpot supports Wireless-N standard, which is much faster than the 802.11g that other mobile hot spots offer. In our trials, it offers very good range (for a device of its size) and decent speeds for the local Wi-Fi clients. And support for a microSD card makes the router a very good solution for those who want to share data on the go. Note that you'll have to turn the "AP Isolation" setting to off using the Web interface for the connected Wi-Fi clients to see/talk one another.
The router gets a little warm when being used, however, so don't leave it inside our pocket or purse.
(Longer bars indicate better performance)
Despite the relatively slow data speeds and the lack of an unlimited data plan, the T-Mobile Sonic 4G Mobile HotSpot's great design, reliability, long battery life, and nifty features make it a very good mobile Internet solution. It would be an excellent one if its data plans were more affordable.