I have the 8k ones and they work fine for most of my needs. As bmwtechguy says tho, the stops fall out easily, though it shouldn't be hard to fix them in there. I find that if you use the ramps on a smooth surface, like inside a garage, the ramps will slip anyways. One solution I've found is to place the ramps on some kind of carpeting. I suppose car mats should work also.
I may look into the 12k lb ones.