I had to use one in France when I was studying there last year, as my accomodation was *very* basic and had no internet connection.
I wasn't all that impressed, to be honest. I was paying around 30 euros a month for 500MB - the most I could get without paying a fortune. I think the prices over here in England are a little more reasonable though lol.
However, the connection would disappear at random times, I'd sometimes not be able to connect at all and sometimes it was sooo slooow.
I was on SFR which is the French equivalent of Vodafone.
I don't think I would get one again (at the moment anyway) - because I rarely use my laptop elsewhere apart from uni and home where there's wifi.