I've not used the system but here's an example of why you should be cautious about entering into such an agreement:
Let's say that you slip on a wet floor in a supermarket. You twist your knee badly and you're off work for a month. A 'no win, no fee' legal firm suggest that you should sue the supermarket company.
However, when it comes to court, the supermarket firm produce video evidence to show that the another customer caused the spill only seconds before you slipped and that, because they could not reasonably have been expected to respond to the spill in such a short time, they are not guilty of negligence. Without evidence of negligence, you'd lose your case.
Your legal firm would honour their pledge and not charge you any fee. However, the supermarket firm might ask the court to order that you should pay their legal costs. For the scenario I've created these might be anywhere in the range of �1000 to �5000. (For a medical claim which spends years going through the courts, one side could run up costs running into millions of pounds). These costs aren't covered by the 'no win, no fee' arrangement and you'd be left with a large bill.
Chris