The T-Mobile one (also from Tesco) is a nice arrangement whcih suits me well. You can use up credits (on PAYG) by buying a day (£2), a week (£7) or a month (£15). This works for me because I only tend to need it on isolated occasions. Also they don't charge if you go over a set limit (which for normal surfing is unlikely anyway). They have a "fair usage policy" and may restrict your access but you won't get any massive shocks. I reckon it is pretty good for me.
The O2 one seems to give access to WiFi hotspots such as The Cloud or BT Openzone, and I may try that next time.