Virgin Mobile has recently launched prepaid data devices in the US. They run on the Sprint PCS network, which is CDMA based. Many seem to prefer over AT&T and T-Mobile.
I purchased the MiFi, which is manufactured by Novatel. Verizon offers the same device but in black instead of silver. It is a WiFi router with a built-in 3G modem.
Unfortunately, while it works, the speeds I've been getting have been atrocious. Average download speeds are claimed to be between 600-1400kbps, but I've not seen more than 600kbps. I've tried this in various locations in WA (Bellevue, Redmond, SeaTac) and some locations in the Los Angeles area. Same issue every time: really really poor data speeds and high latency.
The attraction of course is that this is a no-contract device. you buy it for $150 and can have "unlimited" data for $40 over the period of a month. Now unlimited is rarely unlimited and in this case the small print includes "does not mean unreasonable usage" whatever that actually means.
For comparison, my three year old AT&T 3G USB device consistently gives me over 1Mb down. However, this device is on contract and costs more per month. Also, it isn't a WiFi router, which means it must be tethered to the computer to use it.
We have a Sprint PCS 4G device at work, which I will borrow and perform and A/B test of it versus the MiFi. Right now, though, it isn't looking good for the MiFi!