They are the same as the iPhone International Data Plan. Good for checking email and updating Facebook/Twitter status but that's about it. But I do sign up for it every time I travel overseas.
Since I never use my iPhone as a phone in Europe, I think I'll leave it behind this next trip and just take the iPad. If I can get a local micro SIM so much the better.
Oh, and they do prorate (at least with the iPhone plan) so if you use 10 MB in a week (and then turn it off) they'll refund half.