Yeah, JC is a bomb. I agree with prior comments that the marketing was poor but with so many mediocre reviews, better marketing would have been analogous to lipstick on a pig.
As for Avengers, I think that's a shoo-in for $300-400M domestic.
Sure there is a certain "fanboy" demographic that these super hero films all appeal to. But each fanboy still makes unique see/don't see decisions based upon the film's merits. Personally I've seen most of the X-Men movies in the theater, the first Iron Man, Dark Knight and some of the older Batman films.
But more recently, I saw IM2 in a discount theater 2-3 months after release. Skipped Thor and Green Lantern altogether. Did see Captain America--which was very good--the weekend of its release.
Avengers is approaching event movie-status IMO. The cast alone is so diverse. The names alone (Downey, Evans, Hemsworth, Renner) are enough to draw in wives and girlfriends. I can understand individuals not seeing the appeal in stand-alone films with Thor, CA, IM, etc. But throw them all together and the target audience grows dramatically.
Iron Man 2 earned over $300 mil in the US and wasn't as good as the first. Unless Avengers is a real stink bomb, I don't see any reason why it wouldn't attract the same crowd as IM2, and then some.