giffgaff describes a goody bag as a 'mix of UK minutes, texts, and mobile Internet that lasts for a month'.
And giffgaff tariffs really are unlimited as long as it's personal use and you don't connect the phone to a PC.
giffgaff is owned by O2 and therefore uses the O2 network. My personal experience in London and the South East has been that whilst O2 coverage is average, Vodafone is better, for both voice and data. Of course your experience may be different where you are.
Further information from the giffgaff website.