Well, they're definitley a compromise on proper protection, but better than wearing normal jeans...
I have a couple of pairs, Draggin Jeans and Red Route... The problem for me is that they tend to be shit looking jeans, so you buy them to blend-in, and not look like you've just stepped off a bike, but then you're left wondering why you're wearing these shit high-waisted jeans and wish people knew you'd just stepped off a bike so they don't think you have crap taste in clothes.
They look even worse with the knee armour velcro'd in.
I haven't fallen off in either pair, but it's obvious they won't protect you like leather or proper armoured material stuff...
These days, if I wanna really wear jeans and ride properly, I wear my loosest Diesel Jeans with motorcross knee armour strapped underneath, and lightly padded undershorts (Dainese I think). ... or just wear decent jeans and ride more carefully.