Unless they've released it as a universal app, it's iPhone only -- which means it will work on the iPad, but either only on a screen the size of the iPhone or stretched to fill the iPad screen (and pixilated accordingly).

For me, the web surfing experience is the big one. I used it traveling pretty exclusively this past weekend and once I had activated 3G service, I found myself pulling it out just about everywhere. For me, surfing and doing other "consuming" tasks is far more pleasant on the iPad simply because of the size of the screen.

I haven't been brave enough to travel without my laptop (my company sells Windows software and I could be called on to support it at any time), but on vacation I know I will.