Unfortunately I can help you out here Hemidog. I brought one of the cheaper brands (approx $120) about four years ago. Comfortable, not to heavy and warm in winter. I hit a kangaroo at about 90kph and slid for about 30m on my front. The jeans tore through the denim but the Kevlar literally saved my skin. I was so impressed with the jeans I simply stiched the denim back up and kept on wearing them. 3 months ago I flipped my bike at 145kph and slid on my back for about 60m. While sliding the jeans were forced to move around and in effect give me a wedgy!! Bringing the part of the jeans that is not considered "key area"in contact with the bitumen. End result was not a mark on my legs however some skin of my bum cheek.
So to answer your question, Do they work? hell yes and I'll be buying another pair once the bike is fixed completely, however spend the extra cash unlike I did and get ones with full kelvar supports!!!