At the moment if you just top-up with credit, you can access the internet for free with a fair use policy of 100MB per day. This offer expires on 1st October 2010, after which you will be charged from your available credit at a rate of 50p in total if you use between 2.5MB and 30MB in one day. If you exceed 30MB you will be charged at a rate of 20p per additional MB.
However with a goodybag , unlimited Internet is inclusive and really unlimited, there is no fair use policy. So for medium to heavy users, it really is the best option.