I have two sets of these jump leads and for the money they are pretty good. One set is for my brother's problematic 2.0 HDI car which will only start from two fully charged batteries and the leads are doing the job fine. As mentioned in another review, the cable is terminated through the insulation but I haven't found this to be a problem so far. I would imagine that engines larger than 2.5 litre diesel, these may not stand up to the job though. Over daily use for the last few weeks the insulation has cracked at the termination to one of the clamps but this is to be expected considering the daily use in this cold weather.
Overall, for the average car these are worth having a set in the boot for emergencies. It may save you (or someone else) a lot of hassle.