I recently bought a Novatel U760 USB dongle from Mobi to use their Hele service. It's basically internet access over a 3G cell modem, as I understand it. Lawrence Lee has mentioned it several times here.
I have until Thursday to return it, and after yesterday, there's a good chance that I will. I'm not sure if I have a defective device or what, but it disconnects VERY frequently. I tried it in my new apartment high up on the side of the valley in Manoa (wondering if this matters, though I thought it would help, not hurt -- I get great cell phone reception up there), and the longest it would stay connected last night was about 17 minutes and during the day, maybe as many as 30-60 minutes, but I wasn't paying as much attention then.
When it does work, the throughput and latency are fine with me: up to 150KB/s and local latency as low as 150ms with the avg around 220ms probably (to LavaNet or UH machines), but the frequent disconnects are killer.
I've tried it with a Macbook (Leopard/10.5) and various laptops running Ubuntu 10.04 (using both KPPP and Network Manager), Windows XP, and Windows 7. All with identical behavior. After looking through /var/log/daemon.log in LInux and, additionally, OS X's error console, the device is clearly switching state and appears to be independent of drivers, etc.
Please include time/place that you're using it.
I'm going to go somewhere out of the valley tonight and test more.