ive been doing them since christmas, they are really good!
they are hard work yes, but you dont need to keep up with everyone else. everyone does it at their own level, like the press ups.... some can do five and some can only do one. the point is for you to build it all up and get stronger at it.
did it with a different instructor a few weeks ago and it wasnt as good, she wasnt showing any high impact moves at all when my regular instructor always says go 100% all the way through, dont start easy on anything.
i have noticed improvements though, when i first did them at christmas i couldnt do a single burpee, squat thrust, crab walk or press up on toes, now i can. still need to work on the crab walk but ive noticed a definate improvement in them.