DP opined same last week, and I've done a bit of research and found it's a popular opinion. This article is particularly persuasive. Basically saying that you sweat and your heart races but you don't actually use any muscle.
Said this to my trainer when I had one...her solution was to use the Xtrainer as an interval trainer i.e a ten minute session where you do 30 seconds as fast as you can go - on a high a level as poss, and then 30 seconds recovery on a lower pace/resistance, alternating for the full ten minutes. I found it useful and helped provide a different intensity to interval training on the running machine, power plate exercises, squats, trx etc.
Probably not the best workout option to spend 60 mins on a low intensity programme watching the telly, but useful enough as part of a workout programme.