my GF and I got 3 PAYG phones so we could Skype call one another for nothing.
Seemed like a good idea, but we only got the cursed things to work once, and that was from my parents upstairs to their living room. We managed to get most of our money back on them and are now just using up the credit on the SIM cards (they agreed to refund the handsets but not the credit, so we kept the SIMs). Even on normal calls the reception and signal coverage is diabolical.
Of course YMMV depending on where you are, but I wont be going near 3 again for a few years until theyve had a chance to sort out their signal strength in the Oxon/Bucks/Wilts area.
skype through the 3 network is crap, but their £15 international plan isnt that bad.