After reviewing my data usage I dropped to the 250MB plan. I've never come close to overdoing that, but I only do mail and information style applications when I'm out and about. No movies, or even streaming audio.
I've also chosen to pay for the HotSpot feature on my phone. That's 2GB. This way I get versatility (more devices and two carriers) and a smidgen more total data for about 5 dollars more than the 2GB plan on the iPad.
But I don't work either, at least not at a work place, so I don't have anywhere outside of home that I need an entertainment device.