i have an orange one somwhere. i think the deal was that its £5 a month, but, if you have an orange phone (payg or whatever) its £0 / month.
then you use it, and they just bill you per mb of transfer. its been ages since ive used it, and i think the rate is probably set up for occasional use, rather than general, but, for me, its good, as, on the very rare occasion i need it, i have it (assuming i can find it) and i can just use it, and they bill me (no having to pre-load it with cash)