I have an AT&T Quicksilver card and the cheap, limited data plan for it. I don't use it much, but since it is a 3G device, I thought I'd play with it and the microcell, just to see how it compares to the iPhone data throughput.
To recap, the microcell, unlike the Verizon one, supports both data and voice. If you do a speed test with an iPhone (via the speedtest.net app) through the microcell (this is with the microcell hooked up to 6 MB / 768 kB DSL), you wind up getting about 2 MB down and 50 kbps up. With WiFi, it's 4.5 MB down and 610 kb up.
Data only devices, like the Quicksilver USB "card" still have phone numbers, despite the fact that they can't place or receive actual phone calls. They can send and receive SMS messages, in theory, but unless you buy an SMS messaging plan, it would be expensive, and if you're using the thing, you're on the Internet anyway. It's this phone number that you register with the microcell so that it can connect.
Sure enough, registering the device with the microcell and powercycling it (that is, removing and reinserting it into my laptop) worked. When I hooked up using the GlobeTrotter connect software, the network had changed from "AT&T" to "AT&T MicroCell".
Of course, the unlimited calling plan I have on the microcell does nothing for data. This is something I think AT&T really ought to address - if I'm using a microcell, why shouldn't I get unlimited data for free? This is why I don't normally have the card registered with the microcell - it doesn't do me any good, and frankly, I'd expect to do better just using my own wifi anyway.
And the speed tests seem to back that up. Running the speedtest on my laptop with wifi, I get 5.1 MB down and 610 kb up. This alone suggests that the phone is actually doing pretty well with its own use of wifi. The laptop has a bit of an edge over the phone given that it's got 802.11n and the phone only has g.
Well, the laptop going through the microcell has almost exactly the same results as the iPhone - 2.1 MB down and 60 kbps up.
I'm not sure why data uploads through the microcell are so heavily capped, compared to the native bandwidth available through the DSL connection.
There is a trade-off at work here - albeit only for the iPhone. If you use WiFi, it's a little faster, but if you use 3G, you get a publicly routable address - even with the microcell. For the data card, since the data counter still runs, there's much less incentive to use the Quicksilver through the microcell, however.