One downside to the iPad is that the only mobile carrier it can be used with in the United States is AT&T; although Apple has a perfect right to partner with the carrier of its choice, users who spend a lot of time in locations that don't have very good 3G coverage from AT&T may want to save themselves some aggravation and money by sticking with the WiFi model.
In my field tests of the iPad with WiFi + 3G, I observed good performance overall from AT&T's 3G network, but with some marked exceptions to that rule. Generally, my test pages would load in under a minute from one location to the next at times stretching from 11 a.m. to 2 p.m. But in one set of tests, what was decent performance at a certain spot at 5:15 p.m. turned into abysmal results when I'd moved about 40 feet up the street. Time after time, pages that just a few minutes before had loaded in seconds were still incomplete after five minutes.
Admittedly, that's specific to a busy street of downtown San Francisco, and one expects a degree of network congestion around 5:30 p.m. But I was in a fairly open spot by the standards of any city's downtown, so it may be that the only thing I proved is that AT&T's coverage in the first block or two of Second Street needs improvement. Nevertheless, I'm not sold on Ma Bell's mobile services, not by a long shot.