I still have my regular mountain bike that i will use, but about 25-30 miles is my limit offroad, this just adds another dimension to where i can go, i.e travel further afield and take in more sights/tracks, beach and sandy trails and the assist is only there should you want to turn it on, you dont have to use it, so in theory i will be going further but still doing the same exercise.
A friend bought one who is a similar fitness to me, he did 45 miles the other day and said he definitely felt it the next day, its very easy to see it as "cheating", but it isnt, you still have to pedal to get the output, i was very sceptical until i had a go on one and then it made sense, well it did to me