I haven't read all answers, so sorry if I am repeating someone. As I recall from playing DA2, Bartrand became crazy in act 2, which is three years after they found the lyrium idol. The events in future Redcliff took place just one year forward, which could explain why the companions were not as mentally muddled. Add to that the fact that their willpower was probably a lot stronger than Bartrand's. Some might argue that Bartrand became immediately crazed the minute he touched the idol, but he seemed lucid to me, and him acting so greedily was hardly that much out of character. Also, Hawke touched the idol for about the same amount of time and was not affected.
So I would guess that the effects of the lyrium is dependent on length of exposure, proximity (and whether or not you ingest it) and the character's strength of will.
Do I think it could have been shown better, or the plot better crafted? Yes. Do I think that this makes the quest impossible to enjoy? Absolutely not, it's fascinating to see what Corypheus' world would look like (although I'm admittedly wary of time-travel in general).