Not used them personally, but I know people who have their SIMs. They've been around for a while and there's some discussion here on the forum at:
http://www.prepaidgsm.net/forum/euro...ls-merged.html
They do much of the worlds landlines at 4 or 5 pence/min, and I think they recently added UK into that too. (...Remember the UK is not just England BLC

)
However, UK mobile is higher at 10p/min.