While there seem to be some small differences in pricing for call prices for the IOM cards, Globalsim seems to the only one where you can pay for time (minimum 25 GBP) and get a free card. The only down side I see is that the card dies if you don't top it up every 180 days (as opposed to some of the other cards which will stay alive so long as you use it within a specified period.)

Globalsim has a good offer. If you want we just had a referall link http://www.freeglobalsim.com/?qryAC=27874 . So you can help us too :)