I liked the first series, what probably helped make it even more exciting was that I had previously (briefly) worked with the eventual winner (and he was genuinely a nice bloke).
I think it changed a bit from the second series on when the applicants knew what they were applying for - the adverts to get contestants for the original series (I later realised that I had seen one) were about a life changing adventure but didn't mention the nature of the challlenge, nor that it was for TV.
So now contestants are kind of riffing on the previous series and it leads to more 'TV' unlikeable characters I think.