I have been using the Virgin Mobile MiFi device with my iPad (WiFi) here in Australia since I got it from the states a month ago. There are many advantages, including connecting multiple wifi devices on one access point and data plan. This is great for work and being on the road. Also using the microSD port to easily expand the storage space at hand.
While the mifi is quick to turn on and connect (less then 30 seconds), and the battery power is decent (4hrs at least), it is very occasional IE when away from regular wifi signal and wanting to connect, you ask yourself if it's worth connecting just to google something. For these immediate demands I just use the iPhone. Generally, it's worth connecting the mifi if it's worth getting the iPad out.
I recommend this device where...
You need to connect multiple devices online (or to each other) on the go
You really don't want another data plan for a single device
You plan on upgrading devices often, or only holding onto them for short periods.
You already have wide access to wifi (home, work, etc)