We just got back from the Nevis Range. It is a really tough run, definately far more technical than the average red.
A friend was on a 120mm specialized hardrock and he managed fine.
We also did the world cup downhill, and whilst there were sections of the downhill that were a lot tougher than the red I actually thought the DH and the red were both quite similar in terms of challenge on the whole.
I didnt enjoy either that much though, because as someone else has mentioned, in places the price of failure is significant and that dented my enjoyment . I'm amazed there arent more significant injuries, we did see a guy part way along the boardwalk who had taken a tumble and looked in a pretty bad way
For me, its worth doing once, but I'll not be too bothered about going back.