If you are interested in monthly bundles, they have unlimited texts for £5, 250mins, unlimited texts and unlimited mobile internet for £10 (this is a special offer that expires 31st March 2011, 150mins after that date). Other bundles available, and unlimited text/internet are standard above £5, they also offer free unlimited calls/text to other GiffGaff numbers (topup required every 90 days) so an ideal package for teenagers to use amongst themselves without running up huge bills. You can see their full call charges here.
If you have a second unlocked mobile phone or O2 tied handset and are interested in trying them out order a free Sim card with £5 credit (affliate link) or non affliate order page but not free credit and see for yourself.
They have a very helpful community, and no matter what version of handset you have, they will be able to get you up and running.
And by the way talking about mobile phones, did you know that by the end of 2011 all phones manufactured in the EU will have the same charger? Check this info video.