My day job is Digital Marketing so I'm likely considered a heavy data user.

Last week in London and Bonn - 1 day/night each:

1 - about 100 MB per day.
2 - Corporate email, Apple Maps, Dell Voice (all my voice calls were through this), Yelp, iMessage, LinkedIn, a few YouTube videos, Evernote.
3 - Apple iPhone 4S
4 - No Mifi or tethering
5 - Prepaid T-Mobile SIM in London & blau.de in Germany.
6 - T-Mobile for 10GBP for 500 MB, 10EU for 100 MB (I think) from Blau - after 100 MB the data still works but it "slows down". I got the exceeded limit text but noticed no speed difference afterwards.

I didn't have a chance to get a SIM in card in Paris - forgot to look at CDG and shops were all closed by the time my work day was done.