I have used various dongles for mobile internet from The Phone House ( Yoigo and IData at different times with prepaid credit ) and found all of them VERY expensive for normal internet usage.
The100mb of credit, mentioned by several posters, is not a large amount of data and easily mounts up on casual internet browsing even without actually downloading anything. Worse again, if you use such a system for Skype or internet radio it just eats up credit. Over three weeks recently I spent EUR90 ( EUR 50 for a dongle with 1 Gb credit and two EUR 20 credit top ups where the price is 6 cents per mb.) The only way this kind of system works financially is if you pre-write all e-mails on a word processor and paste the finished result into your e-mail system for sending and switch off when finished. If you receive an e-mail with several pdf attachments, you might have to decide its too expensive to download on the dongle system and, instead, go to a WiFi area as I had to do.