The people I know of who were worst affected are those who go abroad and don't realise just how much the data charges are when they tether their handset with a laptop. For example, one of my colleagues at work got a â‚¬400 bill from Vodafone about two years ago after a business trip to Germany. Apparently, most providers charge between â‚¬5 and â‚¬10 per MB data roaming, even with their mobile broadband dongles! As far as I'm aware of, some networks now charge for just the first few megabytes per day, but even still, data roaming is insanely expensive compared to phone calls.
So far in over 10 years of using a mobile, I've always used pre-pay and generally always manage to keep my usage to roughly â‚¬10 a month, between phone calls, VoIP (as a mobile application) and data usage.