Still a comic than non-comic people will not recognise. Plus Fox will advertise the crap out of the new X-men film, why wouldn't they? They did for X-men 1-3 and Fantastic four 1 and 2 I think it's a safe bet they will for this.
So the not knowing is still not a factor. Hell how many non-comic people had heard of V for Vendetta before this film? Very little yet people still went and saw it, (I personally loved it then got the comic because of it) A new X-men film will sell even if it's a horrible moie I bet a lot still go to see it first week.
But if its labeled as the "X-Men", people will want to see the X-Men they know. Watchmen isn't the X-Men, hence people aren't going into it with expectations of seeing Wolverine or Cyclops. Many would be with a movie titled "X-Men: First Class".
I'm still unsure if First Class will be an origin story with Cyclops, Jean, etc, or feature all new characters. I've been reading online that it seems like certain people interested in making it are pushing for the new character approach.
As soon as kids realize the characters they know aren't in it, they won't be interested. This applies to adults as well. Sure, some people would see it, and opening numbers might be respectable, but I guarantee a HUGE drop-off in the second to third weeks. It just won't hold up. The upcoming Wolverine probably won't have this problem, since its all about the most well known mutant character ever. And the other mutants in that movie (Gambit, Deadpool, etc) are fan favorites, so are an added bonus to fans. BUT, you couldn't make a film and expect big numbers when its based on characters that are even less known than the
supporting characters in an X-Men
spin-off film! The Magneto film might be iffy...he's well known, but I'm not sure if a general audience really cares about the character outside of his role as an X-Men villain (see
Hannibal). And as for First Class...I've said all I need to.