I have completed level 1 day 1 of shred and OMG! It was very hard even though none of the moves were new to me. It is very similar to my bootcamp class (that I haven't been to for 2 weeks)
I think its the combination of the moves they do and the fact that there is no break/recover time between the exercises.
Each session lasts for 20 minutes which isn't long really but it felt long lol
Even the warm up had got my heart beating fast. You do a mixture of cardio like jumping jacks, weights and abs. It was soooooo hard and my legs were like jelly afterwards and the sweat, wow!
You are meant to use weights for some parts but DS borrowed mine and now they are somewhere in his bedroom. I did the moves without the weights and my arms were killing me 
I will do the 2nd day tomorrow either way but reckon I'll be aching in the morning.
My house is a huge tip, the kitchen was spotless yesterday, I don't know how it happens. Its looking a bit better already but it still needs sorting out. I am not in the mood to do it right now though as I have just done 2 lots of washing and hung one lot up.
I have to go and collect DD from her friends house in 1/2 hour, maybe I can pay her to tidy up for me? My kids love earning money, its great.