I use the Verizon MiFi device, and I think it is well worth the money for many reasons:
1) I don't have to do business with AT&T. Nuff said on that subject.
2) MiFi can connect up to 5 devices at once.
3) MiFi is not tied to one device, as the iPad 3G is. For example, my husband can use it with his laptop on trips.
4) MiFi boots within the same amount of time it takes the iPad to boot, so delays are not an issue.
5) Wifi is faster than 3G and video quality is sharper.
6) MiFi is very small (think stack of 6 credit cards), and very lightweight, so not at all hard to carry around.
7) I had a Verizon broadband card for several years, and my unlimited service plan was grandfathered with this device.
To respond to the battery life comparison, it's true that the MiFi battery does not last as long as the iPad's 10-hour battery, but I'm rarely online for 4-5 hours straight where I'm away from both a wireless network AND an electrical outlet. The unit is usable while charging.