I saw it in 3D in a pretty dark theatre, never had a single problem with the film being "too dark", everything was crystal clear and well enough lit. Weird.
I wear prescription glasses, and have never gone to a modern 3D film. I've gone to showings of really old 3D movies in the early 80's, and had trouble with getting the 3D glasses over my prescription glasses.
Since the advent of the "OldMan" Sunglasses that surroundly fit over prescription glasses, I'd imagine that 3D glasses must be similarly available for people who wear prescription glasses, however I've never inquired nor heard about current 3D accommodations for vision-impaired people.
How good is the 3D experiance for prescription glass wearing people nowadays?